Download ODMG OQL User Manual

ODMG OQL
User Manual
Release 5.0 - April 1998
In for m ation in t h is docu m en t is su bject t o ch an ge with ou t
n ot ice an d sh ou ld n ot be con str u ed as a com m it m en t by
O2 Tech n ology.
Th e soft war e descr ibed in th is docu m en t is deliver ed u n der a
licen se or n on disclosu r e agr eem en t.
Th e soft war e can on ly be u sed or copied in accor dan ce wit h t h e
ter m s of th e agr eem en t . It is again st t h e law t o copy t h is
soft war e t o m agn et ic t ape, disk , or an y ot h er m ediu m for an y
pu r pose ot h er t h an t h e pu r ch aser ’s own u se.
Copyr igh t 1992-1998 O2 Tech n ology.
All r igh t s r eser ved. No par t of t h is pu blicat ion can be
r epr odu ced, st or ed in a r et r ieval syst em or t r an sm it t ed in an y
for m or by an y m ean s, elect r on ic, m ech an ical, ph ot ocopy
with ou t pr ior wr itt en per m ission of O2 Tech n ology.
O2 , O 2 En gin e API, O2 C, O2 DBAccess, O2 En gin e, O2 Gr aph ,
O2 Kit , O2 Look , O2 St or e, O2 Tools, an d O2 Web ar e r egist er ed
tr adem ar k s of O2 Tech n ology.
SQL an d AIX ar e r egist er ed t r adem ar k s of In t er n at ion al
Bu sin ess Mach in es Cor por at ion .
Su n , Su n OS, an d SOLARIS ar e r egist er ed t r adem ar k s of Su n
Micr osystem s, In c.
X Win dow System is a r egist er ed tr adem ar k of t h e
Massach u set t s In st itu t e of Tech n ology.
Un ix is a r egist er ed t r adem ar k of Un ix Syst em Labor at or ies, In c.
HPUX is a r egist er ed t r adem ar k of H ewlet t -Pack ar d Com pan y.
BOSX is a r egist er ed t r adem ar k of Bu ll S.A.
IRIX is a r egist er ed t r adem ar k of Siem en s Nixdor f, A.G.
NeXTSt ep is a r egister ed t r adem ar k of t h e NeXT Com pu t er , In c.
Pu r ify, Qu an t ify ar e r egist er ed tr adem ar k s of Pu r e Soft war e In c.
Win dows is a r egist er ed t r adem ar k of Micr osoft Cor por ation .
All oth er com pan y or pr odu ct n am es qu ot ed ar e t r adem ar k s or
r egist er ed t r adem ar k s of t h eir r espect ive t r adem ar k h older s.
Who should read this manual
OQL is an object -or ien t ed SQL-lik e qu er y lan gu age, t h e ODM G
st an dar d. Th is m an u al descr ibes h ow t o u se OQL as an em bedded
fu n ct ion in a pr ogr am m in g lan gu age (e.g. O2 C, C, C++, or J ava) or
in t er act ively as a qu er y lan gu age. It assu m es pr eviou s k n owledge of t h e
O 2 syst em .
Oth er docu m en t s available ar e ou t lin ed, click below.
See O2 Documentation set.
TA BLE O F CONTENT S
Th is m an u al is divided in t o t h e followin g ch apt er s:
1 - In tr odu ct ion
2 - Get t in g St ar t ed
3 - OQL Rat ion ale
4 - OQL Refer en ce
ODM G OQL User Man u al
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TABLE OF CONTENTS
1
Introduction
9
1.1 System Overview......................................................................10
OQL .............................................................................................12
Browser Interface .........................................................................12
1.2 Interactive and embedded query language ...........................14
Interactive OQL ...........................................................................15
Embedded OQL ...........................................................................15
1.3 Manual overview.......................................................................16
2
Getting Started
17
2.1 Basic queries ............................................................................18
Database entry points ...................................................................20
Simple queries..............................................................................20
2.2 Select ... from ... where .........................................................22
Set.................................................................................................22
Join ...............................................................................................24
Path expressions ...........................................................................24
Testing on nil ...............................................................................25
List or array ..................................................................................25
2.3 Constructing results ................................................................27
Creating an object ........................................................................29
2.4 Operators ..................................................................................30
Count ............................................................................................30
Define...........................................................................................30
Element ........................................................................................31
Exists ............................................................................................31
Group by ......................................................................................32
Like ..............................................................................................35
Order by .......................................................................................35
2.5 Set operators ............................................................................36
2.6 Conversions..............................................................................37
List to set ......................................................................................37
Set to list.......................................................................................37
6
ODM G OQL User Man u al
TABLE OF CONTENTS
Flatten .......................................................................................... 38
2.7 Combining operators .............................................................. 38
2.8 Indexes ..................................................................................... 39
Display index............................................................................... 40
2.9 Chapter Summary.................................................................... 41
3
OQL Rationale
43
3.1 The ODMG standard ................................................................ 44
3.2 The ODMG model..................................................................... 44
3.3 OQL by example ...................................................................... 49
Path expressions .......................................................................... 49
Data manipulation ....................................................................... 51
Method invoking ......................................................................... 52
Polymorphism.............................................................................. 53
Operator composition .................................................................. 54
4
OQL Reference
57
4.1 Introduction.............................................................................. 58
4.2 Principles.................................................................................. 58
4.3 Language Definition ................................................................ 59
4.4
Syntactical Abbreviations ...................................................... 82
4.5
OQL BNF.................................................................................. 85
INDEX
ODM G OQL User Man u al
91
7
TABLE OF CONTENTS
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ODM G OQL User Man u al
1
1
I ntr oduction
Con gr at u lat ion s! You ar e n ow a u ser of t h e object-or ien t ed qu er y
lan gu age OQL.
O2 is a r evolu t ion ar y syst em t h at is par t icu lar ly well adapt ed for
developin g lar ge-scale clien t/ ser ver application s in bot h fields of
bu sin ess an d t ech n ical soft war e developm en t .
Th is ch apt er in t r odu ces t h e O2 syst em an d t h e OQL qu er y lan gu age.
Th e ch apt er is divided in t o t h e followin g sect ion s :
• Syst em Over view
• In t er active an d em bedded qu er y lan gu age
• M an u al over view
ODM G OQL User Man u al
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1
Introduction
1.1 System Overview
Th e syst em ar ch it ect u r e of O2 is illu st r at ed in Figu r e 1.1.
External
Interfaces
Development Tools
Standard
Dev. Tools
O2 Dev. Tools
OQL
C
O2 C
C++
Java
Database Engine
O2Engine
O2Store
O2ODBC
O2Corba
O2DB
Access
O2Web
Fi gu r e 1 .1 : O2 Sy st em Ar ch i t ect u r e
Th e O 2 system can be viewed as con sist in g of t h r ee com pon en t s. Th e
Database Engine pr ovides all t h e feat u r es of a D at abase syst em an d an
object -or ien t ed syst em . Th is en gin e is accessed wit h Development Tools ,
su ch as var iou s pr ogr am m in g lan gu ages, O 2 developm en t t ools an d an y
st an dar d developm en t tool. Nu m er ou s External Interfaces ar e pr ovided.
All en com passin g, O 2 is a ver sat ile, por t able, dist r ibu t ed, h igh per for m an ce dyn am ic object -or ien ted dat abase system .
Database En gin e:
10
• O2 St or e
Th e dat abase m an agem en t syst em pr ovides low level
facilit ies, t h r ou gh O2 Stor e API, t o access an d m an age a
dat abase: disk volu m es, files, r ecor ds, in dices an d
t r an sact ion s.
• O2 En gin e
Th e object dat abase en gin e pr ovides dir ect con t r ol of
sch em as, classes, object s an d t r an sact ion s, t h r ou gh
O2 En gin e API. It pr ovides fu ll t ext in dexin g an d sear ch
capabilit ies wit h O 2 Sear ch an d spat ial in dexin g an d
r et r ieval capabilit ies wit h O 2 Spat ial. It in clu des a
Not ificat ion m an ager for in for m in g oth er clien t s
con n ect ed t o t h e sam e O 2 ser ver t h at an even t h as
occu r r ed, a Ver sion m an ager for h an dlin g m u lt iple
object ver sion s an d a Replicat ion API for syn ch r on izin g
m u lt iple copies of an O2 system .
ODM G OQL User Man u al
System Overview :
Pr ogr am m in g Lan gu ages:
O 2 object s m ay be cr eat ed an d m an aged u sin g t h e followin g
pr ogr am m in g lan gu ages, u tilizin g all t h e featu r es available wit h O2
(per sist en ce, collection m an agem en t , t r an sact ion m an agem en t , OQL
qu er ies, et c.)
• C
O2 fu n ct ion s can be in vok ed by C pr ogr am s.
• C++
OD MG com plian t C++ bin din g.
• J ava
OD MG com plian t J ava bin din g.
• O2 C
A power fu l an d elegan t object-or ien t ed fou r t h
gen er at ion lan gu age specialized for easy developm en t
of object dat abase applicat ion s.
• OQL
OD MG st an dar d, easy-t o-u se SQL-lik e object qu er y
lan gu age wit h special feat u r es for dealin g wit h com plex
O2 object s an d m eth ods.
O 2 Developm en t Tools:
• O2 Gr aph
Cr eat e, m odify an d edit an y t ype of object gr aph .
• O2 Look
Design an d develop gr aph ical u ser in ter faces, pr ovides
in t er active m an ipu lation of com plex an d m u ltim edia
object s.
• O2 Kit
Libr ar y of pr edefin ed classes an d m et h ods for fast er
developm en t of u ser applicat ion s.
• O2 Tools
Com plet e gr aph ical pr ogr am m in g en vir on m en t t o
design an d develop O 2 dat abase applicat ion s.
St an dar d Developm en t Tools:
All stan dar d pr ogr am m in g lan gu ages can be u sed wit h st an dar d
en vir on m en t s (e.g. Visu al C++, Su n Spar cwor k s).
Exter n al In t er faces:
• O2 Cor ba
Cr eat e an O2 / Or bix ser ver t o access an O2 dat abase
wit h CORBA.
• O2 DBAccess Con n ect O2 applicat ion s to r elation al dat abases on
r em ot e h ost s an d in vok e SQL st at em en ts.
• O2 OD BC
Con n ect r em ot e ODBC clien t applicat ion s t o O2
dat abases.
• O2 Web
Cr eat e an O2 Wor ld Wide Web ser ver t o access an O2
dat abase th r ou gh t h e in t er n et n etwor k .
ODM G OQL User Man u al
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1
Introduction
OQL
OQL is an object-or ien t ed SQL-lik e qu er y lan gu age. OQL is t h e qu er y
lan gu age of t h e ODM G-9 3 st an dar d 1 . It can be u sed in t wo differ en t
ways eit h er as an em bedded fu n ct ion in a pr ogr am m in g lan gu age or as
an ad h oc qu er y lan gu age.
You can u se OQL as a fu n ct ion called fr om O 2 C, C, C++, Sm allt alk or
J ava, in or der t o m an ipu lat e com plex valu es an d m et h ods. Each
con st r u ct pr odu ces a r esu lt wh ich can t h en be u sed dir ect ly in t h e
pr ogr am m in g lan gu age. Meth ods can be t r igger ed t o m odify t h e
dat abase. You will fin d t h at pr ogr am m in g is easier becau se OQL can
filter valu es u sin g com plex pr edicat es wh ose evalu at ion s ar e opt im ized
by t h e OQL opt im izer in O2 .
OQL can also be u sed in t er act ively as an ad h oc qu er y lan gu age
allowin g dat abase qu er ies fr om bot h t ech n ical an d n on -t ech n ical u ser s.
In t er act ive featu r es in clu de fast an d sim ple br owsin g of t h e dat abase.
Browser Interface
Th e br owser in t er face you see depen ds on t h e oper at in g syst em you ar e
u sin g.
• Unix
In Un ix, th e O2 Look gr aph ical u ser in t er face gen er at or is u sed t o
gen er at e t h e gr aph ical for m of OQL qu er y r esu lts.
Figu r e 1.2 sh ows a t ypical qu er y r esu lt in gr aph ical for m , as gen er ated
by O2 Look .
1. Th e Object Database St an dard: ODMG - 93. At wood, Bar ry, Du h l, Eastm an , Fer ran ,
J ordan , Loom is an d Wade. Edited by R.G.G. Cat tell. © 1996 Mor gan Kau fm an Pu blish er s.
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ODM G OQL User Man u al
System Overview : Browser Interface
Er a ser bu t t on
Object i con s
Fi gu r e 1 .2 : T y p i ca l OQL qu er y r esu l t i n gr a p h i ca l f or m , a s gen er a t ed i n Un i x
In addition t o t h e u su al Motif bu t ton s a gr aph ical qu er y r esu lt h as an
Er aser bu t ton . Click in g on t h e Er aser bu t t on r em oves th e gr aph ical
r esu lt . Th is qu er y r esu lt con sist s of a n u m ber of object s. Each object
h as it s own pop-u p m en u wh ich is displayed by click in g t h e Object icon
u sin g t h e r igh t m ou se bu t t on . Th is pop-u p m en u can be u sed t o access
t h e pu blic m et h ods of each object .
• Windows NT
In Win dows NT, t h e qu er y r esu lt is displayed in a win dow in t ext u al
for m con t ain in g h yper t ext lin k s. Each lin k r epr esen t s a su b-object .
Th e label for a specific lin k m ay be obt ain ed by applyin g t h e title
m et h od t o t h e su b-object r epr esen t ed by t h e lin k .
Click in g on a h yper t ext lin k , with t h e r igh t m ou se bu t t on , r eplaces t h e
con t en t s of t h e win dow wit h a r epr esen t at ion of t h e su b-object
associat ed wit h th e lin k .
Figu r e 1.3 sh ows a t ypical qu er y r esu lt in gr aph ical for m , as gen er at ed
in Win dows NT.
ODM G OQL User Man u al
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1
Introduction
Fi gu r e 1 .3 : Ty p i ca l OQL qu er y r esu l t i n gr a p h i ca l f or m , a s gen er a t ed i n Wi n d ow s NT
Th e br owser sh own in Figu r e 1.3 h as th e followin g bu t t on s:
Ba ck
th is bu tt on displays t h e pr eviou s object .
For w a r d
th is bu tt on displays t h e n ext su b-object. It is on ly valid if
th e Back bu t ton h as been act ivat ed at least on ce.
New Wi n d ow
Th is bu tt on displays t h e cu r r en t object in a n ew win dow.
Each win dow is an in depen den t br owser .
Qu i t
Th is bu tt on closes t h e act ive win dow.
Th e qu er y r esu lt is an object of t h e Per son class, wh ich h as a n am e, an
age an d a spou se. A spou se is also an object of t h e Per son class, an d
t h u s appear s in as a h yper t ext lin k . Left click in g displays t h e spou se
object .
Note
Th e r est of th is m an u al will on ly sh ow gr aph ical displays fr om t h e Un ix
plat for m .
1.2 Interactive and embedded query language
It is becau se OQL is so easy t o u se in ter act ively t h at all k in ds of u ser s
in clu din g n on -t ech n ical u ser s can br owse th e dat abase qu ick ly an d
efficien t ly t o get t h e in for m at ion t h ey wan t . OQL can also be u sed as a
fu n ct ion called fr om C, C++, J ava, O2 C an d O2 En gin e API.
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ODM G OQL User Man u al
Interactive and embedded query language :
Interactive OQL
Th e OQL in ter pr et er can be t r igger ed by t h e query com m an d of O2dba,
O2dsa or O2 sh ells. Th e com m an d in t er pr et er pr om pt s you wit h t h e
followin g m essage:
type your command and end with ^D
To r u n OQL, type:
query
^D
You m u st t ype ^D (Con t r ol - D) on a separ ate lin e. You n ow see:
Query Interpreter
type your query and end with ^D
Type you r qu er y, en din g it wit h ^D.
"this is a query"
^D
Th e an swer is au tom at ically displayed an d th e system r etu r n s to t h e
OQL pr om pt :
type your query and end with ^D
To leave t h e qu er y session type:
^D (or quit)
You ar e n ow back in th e com m an d in t er pr et er an d you see t h e m essage:
type your command and end with ^D
You can also u se OQL in t h e O2 Tools pr ogr am m in g en vir on m en t (Refer
t o t h e O2 Tools User M an u al).
Note
In a Win dows en vir on m en t ^Z (Con t r ol - Z) is u sed in stead of ^D
(Con t r ol - D).
Embedded OQL
An y valid qu er y can be passed fr om O2 C code t o OQL u sin g t h e syst em
su pplied fu n ction o2query. Th is is det ailed in t h e O2 C Refer en ce
m an u al.
Sim ilar ly, you can pass a qu er y t o a C++, C, Sm allt alk or J ava pr ogr am .
Refer to t h e r espect ive m an u als for details.
ODM G OQL User Man u al
15
1
Introduction
Fin ally an OQL fu n ct ion exist s in O2 En gin e an d is descr ibed in t h e
O 2 En gin e API Refer en ce M an u al.
1.3 Manual overview
Th is m an u al is divided u p in t o th e followin g ch apt er s:
• Ch apt er 1 - In t r odu ct ion
Th is ch apt er in t r odu ces t h e O 2 system an d t h e OQL qu er y lan gu age.
It ou t lin es t h e con cept s of t h e ad h oc qu er y lan gu age t h at allows you to
br owse t h e dat abase qu ick ly an d efficien t ly t o get t h e in for m at ion you
wan t , an d th e em bedded qu er y lan gu age t h at you can call fr om in side
you r pr ogr am s.
• Ch apt er 2 - OQL - Get t in g st ar t ed
Th is ch apt er in t r odu ces t h e OQL lan gu age so you can st ar t t o u se OQL
in or der t o obt ain t h e exact in for m ation you wan t fr om you r dat abase.
It descr ibes an d illu st r at es basic an d “select ..fr om ..wh er e” qu er ies,
details h ow t o con st r u ct r esu lts an d descr ibes t h e u se of oper at or s an d
in dexes. To fu lly u n der st an d t h is ch apter , you m u st k n ow t h e ODMG
dat a m odel.
• Ch apt er 3 - OQL Rat ion ale
Th is ch apt er in t r odu ces t h e ODMG stan dar d an d descr ibes t h e ODM G
object m odel. It also gives an exam ple based pr esen t at ion of OQL.
• Ch apt er 4 - OQL Refer en ce
Th is ch apt er con t ain s th e ODMG r efer en ce m an u al for OQL 1.2. It is t h e
sam e as t h e ODMG st an dar d wit h added n ot es an d explan at ion s on h ow
t o u se OQL with O 2 .
For each feat u r e of th e lan gu age, you get th e syn tax, in in for m al
sem an t ics, an d an exam ple. Fin ally, t h e for m al syn t ax is given .
16
ODM G OQL User Man u al
2
G et ti n g S t art ed
2
AN OBJ ECT-ORIENTED DATABASE
QUERY LANGUAGE
So t h at you can obt ain t h e exact in for m ation you wan t fr om you r
dat abase, O2 h as an object or ien t ed database qu er y lan gu age OQL.
OQL is a power fu l an d easy-t o-u se SQL-lik e qu er y lan gu age with special
feat u r es for dealin g wit h com plex objects, valu es an d m eth ods.
Th is ch apt er in t r odu ces t h e OQL lan gu age an d is divided u p in t o t h e
followin g sect ion s:
• Basic qu er ies
• Select ... fr om ... wh er e
• Con str u ctin g r esu lt s
• Oper ator s
• Set oper at or s
• Con ver sion s
• Com bin in g oper at or s
• In dexes
• Ch apter Su m m ar y
To u n der stan d t h is ch apt er you n eed to k n ow th e OD MG dat a m odel 1 .
As an in t r odu ct ion t o t h e data m odel you can r efer t o ch apt er 3 of t h is
m an u al or t h e O 2 C Begin n er ’s Gu ide.
Exper ien ce of SQL, t h ou gh n ot a pr er equ isit e, will facilit at e t h e OQL
lear n in g pr ocess.
1. Th e Object Dat abase Stan dard: ODM G - 93, r elease 1.2. Edit ed by R.G.G. Cat t ell. ©
1996 M or gan Kau fm an Pu blish er s.
ODM G OQL User Man u al
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2
Getting Started
2.1 Basic queries
All t h e exam ples sh own below ar e based on t h e followin g O2 sch em a:
• In O2 C
class o2_set_Employee public type
unique set (Employee)
end;
class o2_list_Client public type
list (Client)
end;
class Company public type
tuple (
name: string,
employees: o2_set_Employee,
clients: o2_list_Client
)
method public title: string
end;
class Client public type
tuple (
name: string,
order: list (tuple ( what: string,
price: real))
)
end;
class Employee public type
tuple (
name: string,
birthday: Date,
position: string,
salary: real)
method age: integer
end;
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ODM G OQL User Man u al
Basic queries
• In C++
class Company {
public:
d_String name;
d_Set<d_Ref<Employee> > employees;
d_List<d_Ref<Client> > clients;
char* title() {return name;}
};
class item { d_String what; double price;};
class Client {
public:
d_String name;
d_Array<item> order;
};
class Employee {
public:
d_String name;
d_Date birthday;
d_String position;
float salary;
int age();
};
Two per sist en t r oot s ar e also defin ed: An object , Globe an d a collect ion
the_employees.
name Globe: Company;
constant name the_employees: o2_set_Employee;
ODM G OQL User Man u al
19
2
Getting Started
Database entry points
To qu er y an y dat abase you n eed var iou s en t r y poin t s.
In O2 t h ese ar e th e n am ed object s an d n am ed valu es.
For exam ple, Globe is an en tr y poin t.
Th e sim plest OQL qu er y calls an en tr y poin t :
Globe
Th is r et u r n s:
In an O2 dat abase, n am ed object s an d valu es can eit h er be valu es of
an y type, or object s of an y class. Con sequ en tly, OQL allows you t o qu er y
valu es or object s of an y type or class.
Note
Th e qu er y r esu lts sh own below ar e all given in th e Un ix gr aph ic for m .
Simple queries
Sim ple qu er ies can in volve differ en t t ypes of valu es:
• Atomic values
With at om ic valu es you can car r y ou t ar ith m et ic calcu lat ion s, e.g.,
2 * 2
Th is is a qu er y wh ich r et u r n s t h e in t eger 4.
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ODM G OQL User Man u al
Basic queries : Simple queries
• Struct values
You can also con sider th e valu e of th e object Globe of class Company as
a st r u ct (or t u ple) valu e wit h t h r ee at tr ibu t es.
Th e on ly oper at ion you can car r y ou t on a st r u ct is extr actin g a field,
e.g.,
Globe.name
Th is r et u r n s t h e n am e of t h e Globe Com pan y.
• List or array values
A list is an or der ed collect ion t h at allows du plicates an d you can
t h er efor e ext r act an y of it s elem en t s if you k n ow t h eir posit ion .
For exam ple, you can ext r act t h e f i r st elem en t of t h e list in clients as
follows.
Globe.clients[0]
In OQL, you cou n t list elem en t s fr om 0.
For OQL, an ar r ay beh aves t h e sam e way as a list .
ODM G OQL User Man u al
21
2
Getting Started
• Call of a method
To apply a m et h od t o an object is a base qu er y, e.g.
Globe.title
Th is applies t h e m et h od title to t h e object Globe an d r et u r n s t h e
r esu lt of t h e m et h od title:
2.2 Select ... from ... where
Th e select from where clau se en ables you to ext r act t h ose elem en t s
m eetin g a specific con dition fr om a col l ec t i on . O2 collect ion s in clu de
set , bag (a m u lt i-set or set wit h du plicates), l i st (an in ser t able an d
dyn am ic ar r ay) or ar r ay .
Th e OQL qu er y h as t h e followin g str u ctu r e:
select:
defin es t h e st r u ct u r e of t h e qu er y r esu lt
from:
in t r odu ces t h e collect ion s again st wh ich t h e qu er y r u n s.
where:
in t r odu ces a pr edicat e t h at filt er s t h e collect ion .
Th is sect ion n ow descr ibes h ow t o u se t h is clau se.
Set
A set is a n on -or der ed collect ion .
Th e m ost fr equ en t qu er y on a set is a f i l t er . Th is con sists of ex t r act i n g
t h e el em en t s of a set wh ich h ave cer t ain ch ar act er istics.
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ODM G OQL User Man u al
Select ... from ... where : Set
For exam ple:
select e
from e in Globe.employees
where e.salary > 200.00
Th is qu er y r et u r n s t h ose em ployees wor k in g at t h e In t er n at ion al Globe
wit h a salar y over 200:
Th e select clau se defin es th e qu er y r esu lt as t h e em ployees an d t h e
from clau se gives t h e set on wh ich t o r u n t h e qu er y. Th e var iable e
r epr esen t s each of it s elem en t s in t u r n . Th e where clau se filt er s t h e
em ployees so t h at t h ose ear n in g m or e t h an 200 ar e ext r act ed.
Th is qu er y t h er efor e bu ilds a collect ion of em ployees.
Th is collect ion is in fact a bag as du plicat es ar e accept ed. You can also
add t h e k eywor d distinct to elim in at e an y du plicat es fr om t h e
r esu lt in g bag an d t h en pr odu ce a t r u e set .
M or eover , you can access fr om e an y at tr ibu t es, e.g. salary an d get a
set of r eal n u m ber s. For exam ple:
select distinct e.salary
from e in Globe.employees
where e.position = "Reporter"
Th is gives a set of t h e salar ies of t h e Repor t er s:
ODM G OQL User Man u al
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2
Getting Started
Join
You can also u se a qu er y t o select fr om m or e t h an on e collect ion :
select e
from e in Globe.employees,
c in Globe.clients
where e.name = c.name
Th is qu er y r et u r n s t h e set of em ployees wh o h ave t h e sam e n am e as a
clien t . If t h er e is a clien t called Ken t an d an em ployee called Ken t , you
see t h e followin g win dow:
Path expressions
Object s ar e r elat ed to ot h er object s, an d in or der t o get t o t h e data it
n eeds, a qu er y can follow var iou s pat h s t h at st ar t fr om an y O2 object or
collect ion . For exam ple,
select distinct ord.what
from cl in Globe.clients,
ord in cl.order
where cl.name = "Haddock"
You obt ain t h e set of wh at t h e clien t (s) called Haddock bou gh t :
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ODM G OQL User Man u al
Select ... from ... where : Testing on nil
Testing on nil
Aft er you r applicat ion h as u pdat ed t h e database, you m ay fin d th at
som e object s ar e n ow equ al t o n il. You can t est for t h is u sin g OQL. For
exam ple, you can t est th at a clien t exist s an d if so, wh ich clien t h as
t h r ee or der s:
select c.name
from c in Globe.clients
where c!=nil and count (c.order) = 3
To sim plify pr ogr am m in g, OQL sk ips n il object s wh en t h ey ar e
en cou n t er ed. If a pat h expr ession con tain s a nil object , a pr edicat e is
always con sider ed false. Th is m ean s t h at th e pr eviou s expr ession can
be r ewr it t en as follows:
select c.name
from c in Globe.clients
where count (c.order) = 3
List or array
A list or an ar r ay is an or der ed collect ion t h at can con tain du plicate
elem en t s.
Sin ce it is or der ed, you m ay ext r act an y of it s elem en t s if you k n ow th eir
position . For exam ple:
Globe.clients[2]
Th is ext r act s t h e t h ir d elem en t of t h e list (t h e fir st elem en t is at posit ion
0).
As with sets you can filt er a list .
ODM G OQL User Man u al
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2
Getting Started
For exam ple: wh at ar e th e n am es of t h e clien t s wh o bu y t h e
In t er n at ion al Globe n ewspaper ?
select e.name
from e in Globe.clients
Th e r esu lt of t h is qu er y is a bag of t h e name of Globe clien ts:
Note
Th e qu er y r et u r n s a bag an d n ot a list . To r et u r n a list , you m u st defin e
an or der . See “Or der by” on page 35.
You can also add t h e k eywor d distinct t o a selection t o elim in at e an y
du plicat es fr om t h e r esu lt in g set .
Note
You can m an ipu lat e ver y com plex st r u ct u r es. A list can be m ade u p of
t u ples wh ich in t u r n can h ave a set at tr ibu t e, et c. Con sequ en t ly, you
h ave access t o all t h e em bedded com pon en t s of an object .
For m or e det ails, r efer t o Sect ion 2.3 for con str u ctin g qu er y r esu lts an d
Sect ion 2.7 for com bin in g oper at or s.
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ODM G OQL User Man u al
Constructing results
2.3 Constructing results
Th e st r u ct u r e of a qu er y r esu lt is ver y oft en im plicit . For exam ple, wh en
you ext r act t h e age field of an em ployee, wh ich is of t ype in t eger , you
obt ain an in t eger . Wh en you filt er a set, bag or list , you obtain a set , bag
or list depen din g on wh at you select .
H owever , you can also con str u ct a qu er y r esu lt wit h an explicit
st r u ct u r e u sin g t h e struct, set, bag, list an d array con st r u ct or s.
For exam ple, u sin g th e str u ct con st r u ct or :
select struct (employee: e.name,
position: e.position,
salary: e.salary)
from e in Globe.employees
or sim ply:
select e.name, e.position, e.salary
from e in Globe.employees
Th is qu er y gives th e n am e, posit ion an d salar y of t h e em ployees at t h e
In t er n at ion al Globe n ewspaper :
ODM G OQL User Man u al
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2
Getting Started
You can u se th e special "*" oper ator t o select all at tr ibu t es of t h e
elem en t s of a collect ion .
For exam ple:
select * from Globe.employees
Not e t h at in t h is exam ple you do n ot n eed t o defin e a var iable wit h
from.
You can also bu ild u p em bedded str u ct u r es sim ply by com bin in g
struct oper at or s.
For exam ple, t o get th e iden t it ies an d salar ies of all t h ose em ployees
wor k in g as r epor ter s an d older t h an 22.
select struct (employee: struct (name: e.name,
age: e.age),
salary: e.salary)
from
e in Globe.employees
where e.position = "Reporter" and
e.age > 22
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ODM G OQL User Man u al
Constructing results : Creating an object
Th is qu er y gives a bag wit h on e elem en t :
Creating an object
You cr eat e valu es u sin g struct, list, array, bag an d set. In OQL,
you can also cr eat e object s u sin g t h e class n am e an d by in it ializin g th e
at t r ibu t es of you r ch oice. An y u n -in it ialized at t r ibu t es ar e set t o th e
defau lt valu e. For exam ple, t o cr eat e an object of t h e class Client:
Client (name: "Trent")
Th is cr eat es a t em por ar y object wit h th e n am e at tr ibu t e in it ialized t o
Trent.
You can th en m ak e t h e object per sist en t in t h e u su al way (r efer t o t h e
O 2 C, C++ an d J ava m an u als). Th e r esu lt of t h is qu er y is t h e n ew object .
An object collect ion can be cr eated in t h e sam e way. For exam ple, u se
t h e followin g qu er y t o cr eat e an o2 _l i st _Cl i en t collect ion .
o2_list_Client (list(Client(name:"John"),
(Client(name:"Jack")))
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2
Getting Started
2.4 Operators
Th is sect ion ou t lin es t h e basic OQL oper ator s you can u se t o qu er y t h e
dat abase.
Count
You can qu er y t h e dat abase u sin g t h e count clau se.
For exam ple, t o fin d ou t h ow m an y em ployees t h er e ar e at t h e
In t er n at ion al Globe n ewspaper :
count (Globe.employees)
Th is qu er y r et u r n s an in t eger .
Oth er aggr egat e oper at or s ar e min, max, sum an d avg.
Define
You can n am e t h e r esu lt of a qu er y u sin g t h e define clau se. For
exam ple,
define MyEmployees as
select e
from e in Globe.employees
where e.name like "Sp*"
Th is n am es t h e r esu lt of t h e qu er y an d n ot th e qu er y itself.
Th e n am e MyEmployees can t h en be u sed in ot h er qu er ies. Nam ed
qu er ies gr eat ly im pr ove t h e legibilit y of com plex qu er ies.
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ODM G OQL User Man u al
Operators : Element
Note
You can on ly r eu se t h ese n am ed qu er ies in t h e sam e qu er y session ,
i.e., u p t o a com m i t or abor t poin t.
Element
Wh en you h ave a set or a bag t h at con t ain s a sin gle elem en t , you
ext r act th e elem en t dir ect ly u sin g t h e element oper at or . For exam ple,
element ( select e
from e in Globe.employees
where e.name = "Tintin")
Th is qu er y gives t h e r esu lt:
Exists
You can add a n ew per sisten t n am e to cover all t h e differ en t com pan ies
t h at exist :
name TheCompanies: list (Company);
You can n ow car r y ou t m or e com plex qu er ies, su ch as select in g wh ich
com pan y h as at least on e em ployee u n der t h e age of 23:
select c.name
from c in TheCompanies
where exists e in c.employees: e.age < 23
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2
Getting Started
Th e an swer is a bag of n am es:
Group by
Th is oper at or gr ou ps t oget h er object s of a collect ion with t h e sam e valu e
for par t icu lar att r ibu t es.
For exam ple,
select *
from e in Globe.employees
group by e.salary
Th is gr ou ps t h e employees by salar y givin g a bag of two-at t r ibu t e
t u ples:
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ODM G OQL User Man u al
Operators : Group by
Th e fir st at tr ibu t e is th e salar y an d is called salary as specified. Th e
secon d is t h e set of object s (em ployees) wit h t h e sam e salar y an d is
called partition.
Th u s, t h e type of th e r esu lt of th is qu er y is:
bag (struct (salary: real,
partition: bag (struct (e:Employee))))
You can wor k on a par t it ion valu e by com pu t in g statistics on each
par tit ion .
Th e followin g qu er y r et u r n s a bag of t wo-att r ibu t e t u ples with t h e salar y
an d t h e n u m ber of em ployees ear n in g each of t h ese salar ies:
select salary, number: count (partition)
from e in Globe.employees
group by e.salary
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2
Getting Started
You get t h e followin g type of win dow:
Fin ally you can filt er t h e r esu lt of gr ou pin g by applyin g pr edicat es on
aggr egat ive oper at ion s. You can select gr ou ps wit h con dit ion s on
aver age, cou n t , su m , m axim u m an d m in im u m valu es of par tit ion s. You
do th is u sin g t h e having clau se.
For exam ple, if you wish t o select on ly gr ou ps wit h m or e t h an on e
salar y:
select salary, number: count (partition)
from e in Globe.employees
group by e.salary
having count (partition) > 1
Th e followin g scr een is displayed.
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ODM G OQL User Man u al
Operators
Like
Th e like oper at or allows you t o t est par t of a ch ar act er st r in g. Th e "*"
ch ar act er stan ds for an y st r in g in clu din g t h e em pt y st r in g.
Th e qu er y:
select distinct e.salary
from e in Globe.employees
where e.name like "Sp*"
r et u r n s t h e salar ies of all em ployees wh ose n am es begin wit h Sp:
Order by
You can obt ain a sor t ed list u sin g t h e order by clau se. For exam ple, t o
sor t t h e em ployees by n am e an d by age:
select e from e in Globe.employees order by e.name, e.age
Th e r esu lt of an order by oper ation is always a l i st , even t h ou gh t h e
sou r ce of th e object s to sor t (t h e set employees, in th is case) m ay be a
set.
Th is qu er y r et u r n s a list of em ployees; t h eir or der is alph abet ical by
n am e, an d t h en by age:
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2
Getting Started
2.5 Set operators
Th e st an dar d set oper at ion s ar e defin ed on set an d bag: union,
intersect (in t er section ) an d except (differ en ce).
You can also wr it e th ese oper at or s as + (u n ion ), * (in t er section ) an d (differ en ce).
You can defin e an ot h er qu er y YourEmployees:
define YourEmployees as
select e
from e in Globe.employees
where e.name = "Tintin"
Now you can com bin e t h e qu er ies by addin g t ogeth er t wo set s:
MyEmployees + YourEmployees
Th e sim ple addit ion (u n ion ) of t h e two set s of em ployees gives you a set
con tain in g t h e an swer :
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ODM G OQL User Man u al
Conversions
Th e pick oper at or is defin ed on a set or a bag. It r et u r n s an elem en t of
t h e collect ion , ch osen ar bit r ar ily.
For exam ple:
pick (MyEmployees)
2.6 Conversions
List to set
To con ver t a list or ar r ay t o a set you u se t h e listtoset oper at or .
Exam ple:
listtoset (Globe.clients) intersect
listtoset (TheCompanies[2].clients)
Set to list
To con ver t a set or bag t o a list you m u st or der it.
ODM G OQL User Man u al
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2
Getting Started
For exam ple:
select e from e in the_employees order by e.salary
r et u r n s a list sor t ed by salar y.
You can also u se "*" t o bu ild a list . Th is avoids a r eal sor t algor it h m an d
sh ou ld be u sed wh en th e fin al or der of t h e list is u n im por t an t .
select e from e in the_employees order by *
r et u r n s a list of all em ployees in r an dom or der .
Flatten
To con ver t a collection of collect ion s in t o a flat t en ed collect ion you u se
t h e flatten oper at or .
For exam ple:
flatten (select distinct c.clients
from c in TheCompanies)
r et u r n s a set of clien t s.
2.7 Combining operators
OQL is a com plet e fu n ct ion al lan gu age in t h at ever y oper at or can be
com bin ed wit h an y ot h er oper at or .
You can u se com bin e an d bu ild u p oper at or s, u n iver sal an d exist en t ial
qu an t ifier s, wild-car d oper at or s, st an dar d set oper at or s as well as list
con cat en ation , or der in g an d gr ou pin g oper at or s on set s, bags an d list s.
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ODM G OQL User Man u al
Indexes : Flatten
For exam ple:
select cl.name, paid: sum (select p.price from p in
cl.order)
from cl in Globe.clients
where count (cl.order) >2
order by sum (select p.price from p in cl.order)
Th is sor t s all t h e clien t s, wit h m or e t h an t wo or der s, by h ow m u ch th ey
h ave paid t o t h e com pan y:
2.8 Indexes
Wh en OQL ext r act s on e or m or e elem en ts fr om a collection u sin g a
specified pr edicate or or der oper at ion , it m u st scan th e wh ole collect ion
t o fin d t h e r equ ir ed elem en t s.
You can im pr ove per for m an ce if th e syst em is able t o dir ect ly access t h e
m atch in g elem en t s. Th is is don e by est ablish in g an in dex on a
collection .
An in dex m aps a k ey t o on e or m or e elem en ts of a n am ed collect ion .
Wh en ever a pr ogr am sear ch es for elem en t s of t h e collect ion u sin g t h e
k ey, t h e syst em u ses th e in dex t o qu ick en t h e sear ch .
Th is en tir e pr ocess is t ot ally t r an spar en t t o you as t h e pr ogr am m er . Th e
absen ce or pr esen ce of an in dex h as n o effect on pr ogr am code, on ly on
syst em per for m an ce.
ODM G OQL User Man u al
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2
Getting Started
Th e ben efit s of in dexes in clu de t h e followin g:
• Com plet e logical an d ph ysical in depen den ce
You do n ot h ave to ch an ge you r qu er y t o u se in dexin g. In dexes ar e
cr eat ed by adm in istr at ion com m an ds.
• High per for m an ce du r in g u se an d m ain t en an ce
Access fr om an in dex m ean s con st an t t im e access ir r egar dless of t h e
size of t h e collect ion .
Exam ple:
• Defin in g an in dex for all em ployees:
create index the_employees on salary;
• Th e followin g qu er y will th en be opt im ized:
select e
from e in the_employees
where e.salary ≥ 1000 and e.salary ≤ 5000
Display index
The "display index" query allows you to see how OQL will use existing indexes in queries you will make. To stop this feature, execute "display index" again.
Note
Please r efer t o t h e Syst em Adm in ist r at ion Gu ide for det ails on h ow t o
cr eat e an d m an age in dexes.
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ODM G OQL User Man u al
Chapter Summary : Display index
2.9 Chapter Summary
Th is ch apt er h as cover ed t h e followin g poin t s:
• Basic queries
To qu er y an y database you n eed var iou s en t r y poin t s. In O2 t h ese ar e
t h e n am ed i n st an ces — i.e. n am ed object s an d n am ed valu es.
Sim ple qu er ies in clu de: callin g an en tr y poin t , applyin g a m et h od t o a
n am ed object , ext r act in g a field, et c.
• Select..from..where
Th e select ... from ... where clau se en ables you to ext r act t h ose
elem en t s m eet in g a specific con dition fr om a list or set .
• Constructing results
Th e st r u ct u r e of a qu er y r esu lt is ver y oft en im plicit. However , you can
also con st r u ct a qu er y r esu lt wit h an explicit st r u ct u r e u sin g t h e
struct, set an d list con st r u ctor s.
• Operators
OQL oper ator s in clu de define, element, order by, count, exists,
group by an d like. Th ey can be com bin ed for com plex qu er ies.
• Indexes
Wh en OQL ext r act s on e or m or e elem en ts fr om a set or list it scan s t h e
wh ole collect ion t o fin d t h e desir ed elem en t s. You can im pr ove
per for m an ce if you t ell th e syst em exact ly wh er e t o look .Th is is don e by
establish in g an in dex on a collect ion . An in dex m aps a k ey t o on e or
m or e elem en t s of a n am ed collect ion .
ODM G OQL User Man u al
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2
42
Getting Started
ODM G OQL User Man u al
OQL Rationale
3
3
Most com m er cial object dat abase syst em s n ow h ave a com m on data
m odel based on t h e OM G object m odel. Th is data m odel is defin ed in t h e
OD MG 93 r epor t . Based on t h is OD MG m odel, t h e qu er y lan gu age OQL
was defin ed an d adopt ed by t h e ODM G gr ou p.
Th is ch apt er is divided as follows:
• Th e ODMG st an dar d
• Th e ODMG m odel
• OQL by exam ple
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3
OQL Rationale
3.1 The ODMG standard
Th e ODMG st an dar d cover s t h e followin g poin t s:
1.
an object m odel
2.
an object defin it ion lan gu age for th is m odel, wit h it s own syn t ax, OD L
or its expr ession t h r ou gh C++ an d Sm allt alk syn t ax
3.
an object qu er y lan gu age for t h is m odel, OQL
4.
a C++ bin din g allowin g C++ pr ogr am s to oper at e on a database
com plian t t o t h e object m odel
5.
a J ava bin din g allowin g J ava pr ogr am s t o oper ate on a dat abase
com plian t t o t h e object m odel
3.2 The ODMG model
Th e ODMG object m odel su ppor t s t h e n ot ion of classes, of object s wit h
at t r ibu t es an d m et h ods, of in h er it an ce an d specializat ion . It offer s t h e
classical t ypes t o deal with st r in g, dat e, t im e, t im e in t er val an d
t im est am p. An d fin ally, it su ppor t s t h e n ot ion s of r el at i on sh i p s an d
col l ect i on s .
ODMG-93 in t r odu ces a set of pr edefin ed gen er ic collection classes:
Set<T>, Bag<T> (a m u lt i-set , i.e., a set wit h r epeat ed elem en t s),
Varray<T> (a var iable size ar r ay), List<T> (a var iable size an d
in ser t able ar r ay).
An object r efer s t o an oth er object t h r ou gh a Ref. A Ref beh aves as a C++
poin t er , bu t wit h m or e sem an tics: it is a per sist en t poin t er bu t
r efer en tial in t egr it y can be expr essed in t h e sch em a an d m ain t ain ed by
t h e syst em . Th is is don e by declar in g t h e r elat ion sh ip as sym m et r ic.
Com bin in g r elat ion sh ips an d collect ion s, an object can r elat e t o m or e
t h an on e object t h r ou gh a r elat ion sh ip. Th er efor e, 1-1 r elat ion sh ips, 1-n
r elat ion sh ips an d n -m r elat ion sh ips can be su ppor ted wit h t h e sam e
gu ar an t ee of r efer en t ial in t egr it y.
ODMG-93 en ables explicit n am es t o be given t o an y object or collect ion .
Fr om a n am e, an applicat ion can dir ectly r et r ieve t h e n am ed object an d
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ODM G OQL User Man u al
The ODMG model
t h en oper at e on it or n avigat e t o oth er object s followin g t h e r elation sh ip
lin k s.
Let u s n ow pr esen t t h e m odel t h r ou gh a com plet e exam ple. We u se h er e
C++ syn t ax for ou r object defin it ion lan gu age, followin g t h e ODM G C++
ODL bin din g (i.e., th e way of defin in g an OD MG sch em a u sin g t h e
st an dar d C++ lan gu age).
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3
OQL Rationale
class Person{
d_String name;
d_Date
d_Set
birthdate;
< d_Ref<Person> > parents
inverse children;
d_List < d_Ref<Person> > children
inverse parents;
d_Ref<Apartment> lives_in
inverse is_used_by;
Meth ods
Person();
int age();
Con st r u ct or : a n ew Per son is bor n
Retu r n s an atom ic t ype
void marriage( d_Ref<Person> spouse);
Th is per son gets a spou se
void birth( d_Ref<Person> child);
Th is per son get s a ch ild
d_Set< d_Ref<Person> > ancestors;;
Set of an cest or s of t h is Per son
virtual d_Set<d_String> activities();
A r edefin able m et h od
};
class Employee: Person{
A su bclass of Per son
float salary;
M et h od
virtual d_Set<d_String> activities();
Th is m eth od is r edefin ed
};
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ODM G OQL User Man u al
The ODMG model
class Student: Person{
A su bclass of Per son
d_String grade;
M et h od
virtual d_Set<d_String> activities();
Th e m et h od is r edefin ed
};
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3
OQL Rationale
class Address{
int number;
d_String street;
};
class Building{
Address address;
A com plex valu e Address em bedded in t h is object
d_List< <d_Ref<Apartment> > apartments
inverse building;
M et h od
d_Ref<Apartment> less_expensive();
};
class Apartment{
int number;
d_Ref<Building> building;
d_Ref<Person> is_used_by
inverse lives_in;
};
d_Set<
d_Ref<Person> >
Persons;
All per son s an d em ployees
d_Set<
d_Ref<Apartment> > Apartments;
Th e Apar t em en t class ext en t
d_Set<
d_Ref<Apartment> > Vacancy;
Th e set of vacan t appar t em en t s
d_List< d_Ref<Apartment> > Directory;
Th e list of appar t em en t s or der ed by t h eir n u m ber of r oom s
};
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ODM G OQL User Man u al
OQL by example : Path expressions
3.3 OQL by example
Let u s n ow t u r n t o an exam ple based pr esen t at ion of OQL. We u se t h e
dat abase descr ibed in t h e pr eviou s sect ion , an d in st ead of t r yin g to be
exh au st ive, we give an over view of t h e m ost r elevan t feat u r es.
Path expressions
As explain ed above, on e can en t er a dat abase t h r ou gh a n am ed object ,
bu t m or e gen er ally as soon as on e get s an object (wh ich com es, for
in st an ce, fr om a C++ expr ession ), on e n eeds a way t o “n avigat e” fr om it
an d r each t h e r igh t dat a on e n eeds. To do t h is in OQL, we u se t h e “.” (or
in differ en t ly “->”) n ot at ion wh ich en ables u s t o go in side com plex
object s, as well as t o follow sim ple r elation sh ips. For in st an ce, given a
Per son p t o k n ow t h e n am e of t h e st r eet wh er e th is per son lives, we u se
t h e followin g OQL qu er y:
p.lives_in.building.adddress.street
Th is qu er y st ar t s fr om a Person, tr aver ses an Apartment, ar r ives in a
Building an d goes in side t h e com plex at t r ibu te of t ype Address to get
t h e st r eet n am e.
Th is exam ple t r eated 1-1 r elation sh ip, let u s n ow look at n -p
r elat ion sh ips. Assu m e we wan t t h e n am es of t h e ch ildr en of t h e per son
p. We can n ot wr it e: p.children.name becau se children is a List of
r efer en ces, so t h e in t er pr etation of t h e r esu lt of t h is qu er y wou ld be
u n defin ed. In t u it ively, t h e r esu lt sh ou ld be a collect ion of n am es, bu t we
n eed an u n am bigu ou s n otation t o t r aver se su ch a m u lt iple r elat ion sh ip
an d we u se t h e select-from-where clau se t o h an dle collect ion s ju st as
in SQL.
select c.name
from c in p.children
Th e r esu lt of t h is qu er y is a valu e of type Bag<St r in g>. If we wan t t o get
a Set , we sim ply dr op du plicat es, lik e in SQL by u sin g t h e distinct
k eywor d.
select distinct c.name
from c in p.children
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OQL Rationale
Now we h ave a m ean s t o n avigat e fr om an y object to an y ot h er object
followin g an y r elation sh ip an d en t er in g an y com plex su bvalu es of an
object .
For in st an ce, we wan t th e set of addr esses of t h e children of each
Person of t h e dat abase. We k n ow t h e collection n am ed Persons
con tain s all t h e per son s of th e dat abase. We h ave n ow to t r aver se t wo
collect ion s: Persons an d Person::children. Lik e in SQL, t h e selectfrom oper at or allows u s t o qu er y m or e t h an on e collection . Th ese
collect ion s th en appear in t h e from par t . In OQL, a collect ion in t h e
from par t can be der ived fr om a pr eviou s on e by followin g a pat h wh ich
st ar t s fr om it , an d t h e an swer is:
select c.lives_in.building.address
from p in Persons,
c in p.children
Th is qu er y in spect s all ch ildr en of all per son s. It s r esu lt is of t h e t ype
Bag<Addr ess>.
• Predicate
Of cou r se, t h e where clau se can be u sed t o defin e an y pr edicat e wh ich
t h en ser ves t o select th e dat a m atch in g t h e pr edicate. For in st an ce, t o
r est r ict t h e pr eviou s r esu lt t o t h e people livin g on Main St r eet , an d
h avin g at least 2 ch ildr en wh o do n ot live in t h e sam e apar tm en t as t h eir
par en ts, t h e qu er y is:
select c.lives_in.building.address
from p in Persons,
c in p.children
where
p.lives_in.building.address.street
= "Main Street" and
count(p.children) >= 2 and
c.lives_in != p.lives_in
• Join
In t h e from clau se, collect ion s wh ich ar e n ot dir ect ly r elat ed can also be
declar ed. As in SQL, t h is allows u s t o com pu t e “join s” between t h ese
collect ion s. For in stan ce, t o fin d t h e people livin g in a st r eet an d h avin g
t h e sam e n am e as t h is st r eet , we do th e followin g: th e Building ext en t
is n ot defin ed in th e sch em a, so we h ave t o com pu t e it fr om t h e
Apartments ext en t . To com pu te t h is in t er m ediat e r esu lt , we n eed a
select-from oper ator again . So t h e join is don e as follows:
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ODM G OQL User Man u al
OQL by example : Data manipulation
select p
from p in Persons,
b in (select distinct a.building
from a in Apartments)
where p.name = b.address.street
Th is qu er y h igh ligh t s t h e n eed for an opt im izer . In t h is case, t h e in n er
select su bqu er y m u st be com pu t ed on ce an d n ot for each per son !
Data manipulation
A m ajor differ en ce bet ween OQL an d SQL is t h at an object qu er y
lan gu age m u st m an ipu lat e com plex valu es. OQL can t h er efor e cr eate
an y com plex valu e as a fin al r esu lt , or in side t h e qu er y as in ter m ediate
com pu tation .
To bu ild a com plex valu e, OQL u ses t h e con st r u ct or s struct, set, bag,
list an d array. For exam ple, t o obt ain t h e addr esses of t h e ch ildr en of
each per son , alon g wit h th e addr ess of th is per son , we u se t h e followin g
qu er y:
select struct(me:
p.name,
my_address:
p.lives_in.building.address,
my_children:
(select struct(
name: c.name,
address:
c.lives_in.building.address)
from c in p.children))
from p in Persons
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OQL Rationale
Th is gives, for each per son , t h e n am e, t h e addr ess, an d t h e n am e an d
addr ess of each ch ild. Th e t ype of t h e r esu lt is a bag of t h e followin g
st r u ct :
struct{
String me;
Address my_address;
Bag<struct{String name;
Address address}> my_children;
}
OQL can also cr eat e com plex object s. For th is pu r pose, it u ses t h e n am e
of a class as a con st r u ct or . At t r ibu tes of t h e object of t h is class can be
in itialized explicit ly by an y valid expr ession .
For in st an ce, t o cr eat e a n ew bu ildin g wit h 2 apar t m en t s, if t h er e is a
t ype n am e in t h e sch em a, called List_apart, defin ed by:
tydedef List<<Ref<Apartment> > List_apart;
t h e qu er y is:
Building(
address:
Address (number: 10,
street: "Main street"),
apartments:
List_apart(list(Apartment(number: 1),
Apartment(number: 2))))
Method invoking
OQL allows m et h od calls wit h or wit h ou t par am et er s an ywh er e t h e
r esu lt t ype of t h e m et h od m at ch es th e expect ed t ype in t h e qu er y. In
case th e m et h od h as n o par am eter , t h e syn t ax for m eth od call is t h e
sam e as for accessin g an at t r ibu te or t r aver sin g a r elat ion sh ip. If t h e
m et h od h as par am eter s, t h ese ar e given bet ween par en t h esis. Th is
flexible syn t ax fr ees t h e u ser fr om k n owin g wh eth er t h e pr oper t y is
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ODM G OQL User Man u al
OQL by example : Polymorphism
st or ed (an at t r ibu t e) or com pu t ed (a m et h od). For in st an ce, t o get t h e
age of t h e oldest ch ild of “Paul”, we wr it e t h e followin g qu er y:
select max(select c.age
from c in p.children)
from p in Persons,
where p.name = "Paul"
Of cou r se, a m et h od can r etu r n a com plex object or a collect ion an d
t h en its call can be em bedded in a com plex pat h expr ession . For
in st an ce, in side a bu ildin g b, t o k n ow wh o in h abits t h ose least
expen sive apar tm en t, we u se th e followin g pat h expr ession :
b.less_expensive.is_used_by.name
Alt h ou gh less_expensive is a m et h od we “t r aver se” it as if it wer e a
r elat ion sh ip.
Polymorphism
A m ajor con t r ibu t ion of object tech n ology is th e possibilit y of
m an ipu lat in g polym or ph ic collect ion s, an d t h an k s t o t h e “lat e bin din g”
m ech an ism , t o car r y ou t gen er ic act ion s on th e elem en t s of t h ese
collection s. For in st an ce, t h e set Persons con t ain s object s of class
Person, Employee an d Student. So far , all th e qu er ies again st t h e
Persons exten t dealt wit h th e t h r ee possible classes of objects of t h e
collection . A qu er y is an expr ession wh ose oper ator s oper at e on t yped
oper an ds. It is cor r ect if t h e type of oper an ds m at ch es t h ose r equ ir ed by
t h e oper at or s. In th is sen se, OQL is a t yped qu er y lan gu age. Th is is a
n ecessar y con dit ion for an efficien t qu er y optim izer . Wh en a
polym or ph ic collect ion is filter ed (for in stan ce Persons), it s elem en t s
ar e st at ically k n own t o be of th at class (for in stan ce Person). Th is
m ean s t h at a pr oper t y of a su bclass (att r ibu t e or m et h od) can n ot be
applied to su ch an elem en t , except in t wo im por tan t cases: lat e bin din g
t o a m et h od, or explicit class in dicat ion .
• Late binding
To list th e act ivit ies of each per son , we u se t h e followin g qu er y:
select p.activities
from p in Persons
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OQL Rationale
activities is a m et h od wh ich h as 3 in car n at ion s, on e for Student,
on e for Employee an d on e for gen er ic Person. Depen din g on t h e k in d of
per son of t h e cu r r en t p, th e r igh t in car n ation is called.
• Class indicator
To go down t h e class h ier ar ch y, a u ser m ay explicit ly declar e th e class of
an object t h at can n ot be in fer r ed st at ically. Th e in ter pr et er t h en h as to
ch eck at r u n t im e, t h at t h is object act u ally belon gs t o t h e in dicated class
(or on e of it s su bclasses).
For exam ple, assu m in g we k n ow t h at on ly “st u den ts” spen d t h eir t im e
in followin g a cou r se of st u dy, we can select t h ose per son s an d get t h eir
gr ade. We explicit ly in dicat e in th e qu er y t h at t h ese per son s ar e
st u den t s:
select ((Student)p). grade
from p in Persons
where
"course of study" in p.activities
Operator composition
OQL is a pu r ely fu n ct ion al lan gu age: all oper at or s can be com posed
fr eely as lon g as t h e t ype syst em is r espect ed. Th is is wh y t h e lan gu age
is so sim ple an d it s m an u al so sh or t . Th is ph ilosoph y is differ en t fr om
SQL, wh ich is an ad-h oc lan gu age wh ose com posit ion r u les ar e n ot
or t h ogon al t o t h e type syst em . Adopt in g a com plet e or th ogon alit y,
m ak es t h e lan gu age easier t o lear n wit h ou t losin g t h e SQL st yle for
sim ple qu er ies. Am on g t h e oper at or s offer ed by OQL bu t n ot yet
in t r odu ced, we can m en t ion t h e set oper at or s (union, intersect,
except), t h e u n iver sal (forall) an d exist en t ial qu an t ifier s (exists), t h e
order by an d group by oper at or s an d t h e aggr egat ive oper at or s
(count, sum, min, max an d avg).
To illu st r at e t h is fr ee com posit ion of oper at or s, let u s wr it e a r at h er
elabor at e qu er y. We wan t t o k n ow th e n am e of t h e str eet wh er e t h e set
of em ployees livin g on t h at st r eet an d h ave t h e sm allest aver age salar y,
com par ed t o t h e sets of em ployees livin g in oth er st r eet s. We pr oceed
st ep by st ep an d u se t h e define OQL in str u ction t o evalu at e t em por ar y
r esu lts.
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ODM G OQL User Man u al
OQL by example : Operator composition
1.
Bu ild t h e ext en t of class Employee (n ot su ppor t ed dir ect ly by t h e
sch em a)
define Employees as
select (Employee) p
from p in Persons
where "has a job" in p.activities
2.
Gr ou p th e em ployees by st r eet an d com pu t e t h e aver age salar y in
each st r eet
define salary_map as
select street,
average_salary: avg (select p.e.salary
from partition p)
from e in Employees
group by e.lives_in.building.address.street
Th e group by oper at or split s t h e em ployees in to par t ition s,
accor din g t o t h e cr it er ion (t h e n am e of t h e st r eet wh er e th is per son
lives). Th e select clau se com pu tes, in each par t it ion , th e aver age of
t h e salar ies of t h e em ployees belon gin g t o t h is par tit ion .
Th e r esu lt of t h e qu er y is of t ype:
Bag<struct{String street;
float average_salary;}>
3.
Sor t th is set by salar y
define sorted_salary_map as
select s from s in salary_map
order by s.average_salary
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OQL Rationale
Th e r esu lt is of t ype:
List<struct{String street;
float average_salary;}>
4.
Now get t h e sm allest salar y (t h e fir st in th e list) an d t ak e t h e
cor r espon din g st r eet n am e. Th is is t h e fin al r esu lt .
sorted_salary_map[0].street
In a sin gle qu er y, we cou ld h ave wr it ten :
(select street,
average_salary: avg (select p.e.salary
from partition p)
from e in (select (Employee) p
from p in Persons
where "has a job" in p.activities)
group by e.lives_in.building.address.street
order by avg (select p.e.salary from partition p))
[0]. street
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ODM G OQL User Man u al
4
O QL Re fer ence
4
Th is ch apt er gives t h e fu ll r efer en cial in for m at ion of t h e object qu er y
lan gu age OQL.
Th e ch apt er is divided in t o t h e followin g sect ion s:
• In t r odu ct ion
• Pr in ciples
• Lan gu age D efin it ion
• Syn t act ical Abbr eviation s
• OQL BNF
Th e in for m ation given below is th e sam e as th at of th e OD MG stan dar d 1
wit h n ot es added on h ow t o u se th is lan gu age wit h O2 .
1. Th e Object Dat abase St an dar d: ODMG - 93. At wood, Du h l, Ferr an , Loom is an d Wade.
Edit ed by R.G.G. Catt ell. © 1996 M organ Kau fm an Pu blish ers.
ODM G OQL User Man u al
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4
4.1 Introduction
In t h is ch apt er , a for m al an d com plet e defin it ion of t h e lan gu age is
given . For each featu r e of t h e lan gu age, we give t h e syn t ax, it s
sem an t ics, an d an exam ple. Alt er n at e syn t ax for som e feat u r es ar e
descr ibed in Sect ion 4.4, wh ich com plet es OQL in or der t o accept an y
syn t act ical for m of SQL.
Th e ch apter en ds with t h e for m al syn t ax wh ich is given in Sect ion 4.5
4.2 Principles
Ou r design is based on t h e followin g pr in ciples an d assu m pt ion s:
• OQL r elies on t h e ODMG object m odel.
• OQL is a su per set of t h e st an dar d SQL par t wh ich allows you t o qu er y
a dat abase. Th u s, an y select SQL sen t en ce wh ich r u n s on r elat ion al
t ables, wor k s with t h e sam e syn t ax an d sem an t ics on collect ion s of
ODMG object s. Ext en sion s con cer n Object Or ien ted n ot ion s, lik e
com plex object s, object iden t it y, pat h expr ession , polym or ph ism ,
oper at ion in vocat ion , lat e bin din g et c...
• OQL pr ovides h igh -level pr im it ives t o deal wit h set s of object s bu t
does n ot r est r ict it s at ten tion t o t h is collection con st r u ct . Th u s, it
also pr ovides pr im it ives t o deal wit h str u ct u r es, list s, ar r ays, an d
t r eat s all su ch con st r u ct s wit h t h e sam e efficien cy.
• OQL is a fu n ct ion al lan gu age wh er e oper at or s can fr eely be com posed,
as soon as t h e oper an ds r espect th e t ype syst em . Th is is a
con sequ en ce of t h e fact t h at t h e r esu lt of an y qu er y h as a t ype wh ich
belon gs t o t h e ODMG t ype m odel, an d t h u s can be qu er ied again .
• OQL is n ot com pu t at ion ally com plet e. It is an easy to u se qu er y
lan gu age wh ich pr ovides easy access t o an object database.
• Based on t h e sam e t ype syst em , OQL can be in vok ed dir ect ly fr om
wit h in pr ogr am m in g lan gu ages for wh ich an OD MG bin din g is
defin ed, e.g., C++. Con ver sely, OQL can in vok e oper at ion s
pr ogr am m ed in t h ese lan gu ages.
• OQL does n ot pr ovide explicit u pdate oper at or s bu t r at h er can in vok e
oper at ion s defin ed on objects for th at pu r pose, an d th u s does n ot
br each t h e sem an t ics of an Object D at abase wh ich , by defin ition , is
m an aged by t h e "m et h ods" defin ed on t h e objects.
• OQL pr ovides declar at ive access to object s. Th u s OQL qu er ies can be
easily opt im ized by vir t u e of t h is declar at ive n atu r e.
• Th e for m al sem an t ics of OQL can easily be defin ed.
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ODM G OQL User Man u al
Language Definition : Query Program
4.3 Language Definition
OQL is an "expr ession " lan gu age. A qu er y expr ession is bu ilt fr om t yped
oper an ds com posed r ecu r sively by oper ator s. We will u se th e t er m
expression t o design at e a valid qu er y in t h is sect ion .
4.3.1 Query Program
A qu er y pr ogr am con sist s of a (possibly em pt y) set of qu er y defin it ion
expr ession s followed by an expr ession , wh ich is evalu at ed as t h e qu er y
it self. Th e set of qu er y defin it ion expr ession s is n on r ecu r sive (alt h ou gh
a qu er y m ay call an oper ation wh ich issu es a qu er y r ecu r sively).
For exam ple:
define jones as select distinct x from Students x
where x.name = "Jones";
select distinct
student_id from jones
Th is defin es th e set jones of st u den t s n am ed J on es, an d evalu ates t h e
set of t h eir student_ids.
O2 note
Wit h t h e O2 qu er y in t er pr et er you u se CTRL -D (on Un ix) or CTRL-Z (On
Win dows) bet ween t wo qu er ies r at h er t h at ";".
4.3.2 Named Query Definition
If q is an iden t ifier an d e is a qu er y expr ession , t h en define q as e is
a qu er y defin it ion expr ession wh ich defin es t h e qu er y wit h n am e q.
Exam ple:
define Does as select x from Student
x
where x.name ="Doe"
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Th is st at em en t defin es Does as a qu er y r et u r n in g a bag con tain in g all t h e
st u den t s wh ose n am e is Doe.
define Doe as element(select x from Student
x
where x.name="Doe")
Th is st at em en t defin es Doe as a qu er y wh ich r et u r n s t h e st u den t wh ose
n am e is D oe (if t h er e is on ly on e, ot h er wise an except ion is r aised).
O2 note
• def i n e oper at ion is available on ly wit h t h e in ter act ive qu er y
in t er pr et er . It h as n o m ean in g for OQL em bedded in pr ogr am m in g
lan gu ages (C++, Sm allt alk , O 2 C) becau se st an dar d pr ogr am m in g
lan gu age var iables can be u sed for t h at pu r pose.
• A defin ed n am e is valid u p t o t h e n ext com m it or abor t
• You can get t h e list of cu r r en t defin ed qu er ies by t ypin g t h e qu er y:
d i spl ay qu er i es
4.3.3 Elementary Expressions
4.3.3.1 Atomic Literals
If l is an at om ic lit er al, t h en l is an expr ession wh ose valu e is t h e lit er al
it self.
Lit er als h ave t h e u su al syn tax:
• Object Lit er al: n il
• Boolean Liter al: false, tr u e
• In t eger Lit er al: sequ en ce of digit s, e.g, 27
• Float Lit er al: m an t issa/ expon en t . Th e expon en t is opt ion al, e.g., 3.14
or 314.16e-2
• Ch ar act er Lit er al: ch ar act er bet ween sim ple qu ot es, e.g., ’z’
• St r in g Lit er al: ch ar act er st r in g bet ween dou ble qu ot e, e.g.,"a st r in g"
4.3.3.2 Named Objects
If e is a n am ed object , t h en e is an expr ession . It defin es th e en t it y
at t ach ed t o th e n am e.
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ODM G OQL User Man u al
Language Definition : Construction Expressions
Exam ple:
Students
Th is qu er y defin es t h e set of st u den t s. We h ave assu m ed h er e t h at t h e
n am e Students exist s wh ich cor r espon ds t o t h e ext en t of objects of t h e
class Student.
4.3.3.3 Iterator Variable
If x is a var iable declar ed in a fr om par t of a select -fr om -wh er e..., t h en x
is an expr ession wh ose valu e is t h e cu r r en t elem en t of t h e it er at ion over
t h e cor r espon din g collect ion .
4.3.3.4 Named Query
If define q as e is a qu er y defin it ion expr ession , t h en q is an
expr ession .
Exam ple:
Doe
Th is qu er y r et u r n s t h e st u den t wit h n am e Doe. It r efer s t o t h e qu er y
defin ition expr ession declar ed in Sect ion 4.3.2.
4.3.4 Construction Expressions
4.3.4.1 Constructing Objects
If t is a t ype n am e, p1, p2, ...,pn ar e pr oper t ies of t, an d e1, e2,
...,en ar e expr ession s, th en t ( p1: e1..., pn: en ) is an expr ession .
Th is defin es a n ew object of t ype t wh ose pr oper t ies p1, p2, ...,pn
ar e in it ialized wit h t h e expr ession s e1, e2, ...,en. Th e t ype of ei m u st
be com pat ible wit h t h e t ype of pi.
If t is a t ype n am e of a collection an d e is a collect ion lit er al, t h en t(e)
is a collection object . Th e t ype of e m u st be com pat ible wit h t.
Exam ples:
Employee (name: "Peter", boss: Chairman)
Th is cr eat es a m u t able Em ployee object .
ODM G OQL User Man u al
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vectint (set(1,3,10))
Th is cr eates a m u t able set object (assu m in g t h at vect i n t is t h e n am e of
a class wh ose type is Bag<in t>).
4.3.4.2 Constructing Structures
If p1, p2, ...,pn ar e pr oper t y n am es, an d e1, e2, ..., en ar e
expr ession s, t h en
struct (p1: e1, p2: e2, ..., pn: en)
is an expr ession . It defin es t h e st r u ct u r e t ak in g valu es e1, e2, ..., en
on pr oper ties p1, p2, ...,pn.
Not e t h at t h is dyn am ically cr eates an in stan ce of t h e t ype struct(p1:
t1, p2: t2, ..., pn: tn) if ti is t h e t ype of ei.
Exam ple:
struct(name: "Peter", age: 25);
Th is r et u r n s a st r u ct u r e with t wo at t r ibu t es name an d age t ak in g
r espect ive valu es Peter an d 25.
See also abbr eviat ed syn tax in som e con t ext s, in Sect ion 4.4.1.
4.3.4.3 Constructing Sets
If e1, e2, ..., en ar e expr ession s, t h en set(e1, e2, ..., en) is an
expr ession . It defin es t h e set con tain in g t h e elem en t s e1, e2, ..., en.
It cr eat es a set in st an ce.
Exam ple:
set(1,2,3)
Th is r et u r n s a set con sist in g of t h e t h r ee elem en t s 1, 2, an d 3.
4.3.4.4 Constructing Lists
If e1, e2, ..., en ar e expr ession s, t h en
list(e1, e2, ..., en)
or sim ply
(e1, e2, ..., en)
ar e expr ession s. Th ey defin e th e list h avin g elem en t s e1, e2, ..., en.
Th ey cr eat e a list in st an ce.
If m in , m ax ar e t wo expr ession s of in t eger or ch ar act er t ypes, su ch t h at
m in < m ax, t h en
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list(min .. max)
or sim ply
(min .. max)
ar e expr ession s wh ose valu e is: list (m in , m in +1, ... m ax-1, m ax)
Exam ple:
list(1,2,2,3)
Th is r et u r n s a list of fou r elem en t s.
Exam ple:
list(3 .. 5)
Th is r et u r n s t h e list (3,4,5)
O2 note
In O2 t h e k eywor d l i st is m an dat or y.
4.3.4.5 Constructing Bags
If e1, e2, ..., en ar e expr ession s, t h en bag(e1, e2, ..., en) is an
expr ession . It defin es t h e bag h avin g elem en t s e1, e2, ..., en. It
cr eat es a bag in st an ce.
Exam ple:
bag(1,1,2,3,3)
Th is r et u r n s a bag of five elem en t s.
4.3.4.6 Constructing Arrays
If e1, e2, ..., en ar e expr ession s, t h en array(e1, e2, ..., en) is
an expr ession . It defin es an ar r ay h avin g elem en t s e1, e2, ..., en. It
cr eat es an ar r ay in st an ce.
Exam ple:
array(3,4,2,1,1)
Th is r et u r n s an ar r ay of five elem en t s.
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4.3.5 Atomic Types Expressions
4.3.5.1 Unary Expressions
If e is an expr ession an d <op> is a u n ar y oper at ion valid for th e t ype of
e, t h en <op> e is an expr ession . It defin es t h e r esu lt of applyin g <op> to
e.
Ar it h m et ic u n ar y oper at or s ar e: +, -, abs
Boolean u n ar y oper at or is: n ot.
Exam ple:
not true
Th is r et u r n s false.
4.3.5.2 Binary Expressions
If e1 an d e2 ar e expr ession s an d <op> is a bin ar y oper at ion , t h en
e1<op>e2 is an expr ession . It defin es th e r esu lt of applyin g <op> t o e1
an d e2.
Ar it h m et ic in t eger bin ar y oper at or s ar e: +, -, *, / , m od (m odu lo)
Float in g poin t bin ar y oper at or s ar e:
+, -, *, /
Relat ion al bin ar y oper at or s ar e:
=, !=. <. <=, >, >=
Th ese oper at or s ar e defin ed on all at om ic t ypes.
Boolean bin ar y oper at or s ar e:
an d, or
Exam ple:
count(Students) - count(TA)
Th is r et u r n s t h e differ en ce bet ween t h e n u m ber of st u den t s an d th e
n u m ber of TAs.
4.3.5.3 String Expressions
If s1 an d s2 ar e expr ession s of type st r in g, th en
s1 || s2, an d
s1 + s2
ar e equ ivalen t expr ession s of type st r in g wh ose valu e is t h e
con cat en ation of t h e t wo st r in gs.
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O2 note
In O2 t h e oper ator | | is n ot accept ed. To con cat en at e 2 st r in gs u se "+".
If c is an expr ession of type ch ar act er , an d s an expr ession of t ype
st r in g, t h en
c in s
is an expr ession of t ype boolean wh ose valu e is t r u e if t h e ch ar act er
belon gs to t h e st r in g, else false.
If s is an expr ession of type st r in g, an d i is an expr ession of t ype
in t eger , t h en
s[i]
is an expr ession of t ype ch ar act er wh ose valu e is t h e i+1t h ch ar act er of
t h e st r in g.
If s is an expr ession of type st r in g, an d low an d up ar e expr ession s of
t ype in t eger , t h en
s[low:up]
is an expr ession of t ype st r in g wh ose valu e is t h e su bst r in g of s fr om th e
low+1 t h ch ar act er u p t o t h e u p+1 t h ch ar acter .
If s is an expr ession of type st r in g, an d pattern a st r in g lit er al wh ich
m ay in clu de t h e wildcar d ch ar act er s: "?" or "_", m ean in g an y ch ar act er ,
an d "*" or "%", m ean in g an y su bst r in g in clu din g an em pt y su bst r in g,
t h en
s like pattern
is an expr ession of t ype boolean wh ose valu e is t r u e if s m at ch es t h e
pat t er n , else false.
Exam ple:
’a nice string’
like ’%nice%str_ng’
is t r u e.
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O2 note
In O2 t h e on ly su ppor ted wildcar d is "*".
4.3.6
Object Expressions
4.3.6.1 Comparison of Mutable Objects
If e1 an d e2 ar e expr ession s wh ich den ot e m u t able object s (objects wit h
iden tit y) of t h e sam e t ype, t h en
e 1 = e2
an d
e1 != e2
ar e expr ession s wh ich r et u r n a boolean . Th e secon d expr ession is
equ ivalen t t o not(e1 = e2).
e1 = e2 is t r u e if th ey design at e t h e sam e object .
Exam ple:
Doe = element(select s from Students s
where s.name = "Doe")
is tr u e.
4.3.6.2 Comparison of Immutable Objects
If e1 an d e2 ar e expr ession s wh ich den ot e im m u t able object s (lit er als) of
t h e sam e t ype, t h en
e1 = e2
an d
e1 != e2
ar e expr ession s wh ich r et u r n a boolean . th e secon d expr ession is
equ ivalen t t o
not(e1 = e2).
e1 = e2 is t r u e if th e valu e e1 is equ al t o t h e valu e e2.
4.3.6.3 Extracting an Attribute or Traversing a Relationship from an Object
If e is an expr ession , if p is a pr oper t y n am e, t h en e->p an d e.p ar e
expr ession s. Th ese ar e alter n at e syn t ax t o extr act t h e pr oper t y p of an
object e.
If e h appen s t o design at e a delet ed or a n on exist in g object , i.e. nil, an
at t em pt t o access t h e at t r ibu t e or t o t r aver se t h e r elat ion sh ip r aises an
except ion . However , a qu er y m ay t est explicit ly if an object is differ en t
fr om nil befor e accessin g a pr oper t y.
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Exam ple:
Doe.name
Th is r et u r n s Doe.
Exam ple:
Doe->spouse != nil and Doe->spouse->name = "Carol"
Th is r et u r n s true, if Doe h as a spou se wh ose n am e is Car ol, or else
false.
O2 note
Accor din g to a r ecen t evolu t ion of t h e ODM G st an dar d, OQL does n ot
n ow r aise an except ion wh en it t r aver ses a pat h wh ich con t ain s a nil.
In st ead of t h is, a pr edicat e in volvin g su ch a pat h is always false. Th is
m ean s th at OQL n ow sk ips su ch elem en t s an d th u s t h e explicit test t o
nil is n ot yet m an dat or y.
4.3.6.4 Applying an Operation to an Object
If e is an expr ession , if f is an oper at ion n am e, t h en
e->f
an d
e.f
ar e expr ession s. Th ese ar e alt er n ate syn t ax t o apply on oper at ion on an
object . Th e valu e of t h e expr ession is t h e on e r et u r n ed by t h e oper ation
or else t h e object nil, if t h e oper at ion r et u r n s n ot h in g.
Exam ple:
jones->number_of_students
Th is applies th e oper at ion number_of_students to jones.
4.3.6.5 Applying an Operation with Parameters to an Object
If e is an expr ession , if e1, e2 , ..., en ar e expr ession s, if f is an
oper ation n am e, t h en
e->f(e1, e2, ..., en) an d
e.f(e1, e2, ..., en)
ar e expr ession s t h at apply oper at ion f wit h par am et er s e1, e2, ...,
en to object e. Th e valu e of t h e expr ession is t h e on e r et u r n ed by th e
oper ation or else t h e object n il, if t h e oper at ion r et u r n s n ot h in g.
In both cases, if e h appen s t o design at e a deleted or a n on exist in g object ,
i.e. nil, an at t em pt to apply an oper at ion t o it r aises an except ion .
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H owever , a qu er y m ay t est explicit ly if an object is differ en t fr om nil
befor e applyin g an oper at ion .
Exam ple:
Doe->apply_course("Maths", Turing)->number
Th is qu er y calls t h e oper ation apply_course on class St u den t for t h e
object D oe. It passes t wo par am et er s, a st r in g an d an object of class
Pr ofessor . Th e oper at ion r et u r n s an object of t ype Cou r se an d t h e qu er y
r et u r n s t h e n u m ber of t h is cou r se.
4.3.6.6 Dereferencing an Object
If e is an expr ession wh ich den ot es an object wit h iden t ity (a m u t able
object ), t h en *e is an expr ession wh ich deliver s t h e valu e of t h e object (a
lit er al).
Exam ple:
Given t wo var iables of t ype Per son , p1 an d p2, th e pr edicat e
p1 = p2
is tr u e if bot h var iables r efer t o t h e sam e object , wh ile
*p1 =*p2
is t r u e if t h e objects h ave th e sam e valu es, even if t h ey ar e n ot th e sam e
object s.
4.3.7 Collections Expressions
4.3.7.1 Universal Quantification
If x is a var iable n am e, e1 an d e2 ar e expr ession s, e1 den ot es a collect ion
an d e2 a pr edicat e, t h en
for all x in e1: e2
is an expr ession . It r et u r n s true if all t h e elem en t s of collect ion e1 sat isfy
e2 an d false ot h er wise.
Exam ple:
for all x in Students: x.student_id > 0
Th is r et u r n s true if all t h e object s in t h e Students set h ave a posit ive
valu e for t h eir student_id att r ibu t e. Ot h er wise it r et u r n s false.
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4.3.7.2 Existential Quantification
If x is a var iable n am e, if e1 an d e2 ar e expr ession s, e1 den ot es a
collection an d e2 a pr edicate, t h en
exists x in e1: e2
is an expr ession . It r et u r n s true if t h er e is at least on e elem en t of
collection e1 t h at sat isfies e2 an d false ot h er wise.
Exam ple:
exists x in Doe.takes: x.taught_by.name = "Turing"
Th is r et u r n s true if at least on e cou r se Doe t ak es is t au gh t by som eon e
n am ed Tu r in g.
If e is a collect ion expr ession , t h en
exists(e)
an d
unique(e)
ar e expr ession s wh ich r et u r n a boolean valu e. Th e fir st on e r et u r n s t r u e
if t h er e exist s at least on e elem en t in t h e collect ion , wh ile th e secon d on e
r et u r n s t r u e, if t h er e exists on ly on e elem en t in t h e collect ion .
Not ice t h at t h ese oper at or s allow th e accept an ce of t h e SQL syn t ax for
n est ed qu er ies su ch as:
select ... from col where exists ( select ... from col1
where predicate)
Th e n est ed qu er y r et u r n s a bag to wh ich t h e oper at or exist s is applied.
Th is is of cou r se t h e t ask of an opt im izer t o r ecogn ize t h at it is u seless to
com pu te effect ively t h e in t er m ediat e bag r esu lt .
O2 note
In O 2 t h ese t wo last oper at ion s ar e n ot su ppor ted. On ly t h e for m "ex i st s
x i n e1 : e2 " is valid.
4.3.7.3 Membership Testing
If e1 an d e2 ar e expr ession s, e2 is a collect ion , e1 h as th e t ype of it s
elem en t s, th en
e1 in e2
is an expr ession . It r et u r n s true if elem en t e1 belon gs to collect ion e2.
Exam ple:
Doe in Does
Th is r et u r n s true.
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4.3.7.4 Aggregate Operators
If e is an expr ession wh ich den ot es a collection , if <op> is an oper at or
fr om {min, max, count, sum, avg}, t h en <op>(e) is an expr ession .
Exam ple:
max (select salary from Professors)
Th is r et u r n s t h e m axim u m salar y of t h e Pr ofessor s.
4.3.8 Select From Where
If e1, e2, ..., en ar e expr ession s wh ich den ot e collect ion s, an d x1, x2,
..., xn ar e var iable n am es, if e’ is an expr ession of t ype boolean , an d
if projection is an expr ession or t h e ch ar act er *, t h en
select projection from e1 as x1, e2 as x2 .., en
as xn where e’
an d
select distinct projection from e1 as x1, e2 as x2 ..., en
xn where e
as
ar e expr ession s.
Th e r esu lt of t h e qu er y is a set for a select distinct or a bag for a
select.
If you assu m e e1, e2, ..., en ar e all set an d bag expr ession s, t h en
t h e r esu lt is obtain ed as follows: t ak e t h e car t esian pr odu ct 1 of th e set s
e1, e2, ..., en; filt er t h at pr odu ct by expr ession e’ (i.e., elim in at e fr om
t h e r esu lt all object s th at do n ot sat isfy boolean expr ession e’); apply
t h e projection t o each on e of t h e elem en t s of t h is filt er ed set an d get t h e
r esu lt. Wh en t h e r esu lt is a set (dist in ct case) du plicat es ar e
au tom at ically elim in at ed.
Th e situ at ion wh er e on e or m or e of t h e collect ion s e1, e2, ..., en is
an in dexed collect ion is a lit t le m or e com plex. Th e select oper at or fir st
con ver t s all th ese collect ion s in to set s an d applies t h e pr eviou s
oper at ion . Th e r esu lt is a set (dist in ct case) or else a bag. So, in t h is case,
we sim ply t r an sfor m each of t h e ei’s in t o a set an d apply t h e pr eviou s
defin it ion .
4.3.8.1 Projection
Befor e t h e pr oject ion , t h e r esu lt of t h e it er at ion over t h e from var iables is
of t ype
1. Th e car tesian pr odu ct bet ween a set an d a bag is defin ed by fir st con vert in g t h e set in t o
a bag, an d t h en get tin g t h e resu ltin g bag wh ich is t h e cart esian pr odu ct of th e t wo bags.
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bag< struct(x1: type_of(e1 elements), ... xn: type_of(en
elements)) >
Th e pr oject ion is con st r u ct ed by an expr ession wh ich can th en r efer
im plicit ly t o th e "cu r r en t " elem en t of th is bag, u sin g t h e var iables xi. If
for ei n eit h er explicit n or im plicit var iable is declar ed, t h en xi is given an
in t er n al syst em n am e (wh ich is n ot accessible by t h e qu er y an yway).
By con ven t ion , if t h e pr oject ion is sim ply "*", t h en t h e r esu lt of t h e
select ion is t h e sam e as t h e r esu lt of th e it er at ion .
If t h e pr oject ion is "dist in ct *", t h e r esu lt of t h e select is t h is bag
con ver t ed in t o a set .
In all ot h er cases, t h e pr oject ion is explicitly com pu t ed by t h e given
expr ession .
Exam ple:
select couple(student: x.name, professor: z.name)
from Students as x,
x.takes as y,
y.taught_by as z
where z.rank = "full professor"
Th is r et u r n s a bag of object s of t ype couple givin g st u den t n am es an d
t h e n am es of t h e fu ll pr ofessor s fr om wh ich t h ey t ak e classes.
Exam ple:
select
*
from Students as x,
x.takes as y,
y.taught_by as z
where z.rank = "full professor"select
*
Th is r et u r n s a bag of st r u ct u r es, givin g for each st u den t "object ", t h e
sect ion object followed by t h e st u den t an d t h e fu ll pr ofessor "object "
t each in g in t h is sect ion :
bag< struct(x: Student, y: Section, z: Professor) >
4.3.8.2 Iterator Variables
A var iable, xi, declar ed in t h e from par t r an ges over th e collect ion ei an d
t h u s h as t h e t ype of t h e elem en t s of t h is collect ion . Su ch a var iable can
be u sed in an y oth er par t of t h e qu er y t o evalu at e an y oth er expr ession s
(see t h e Scope Ru les in Section 4.3.15). Syn t actical var iat ion s ar e
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accept ed for declar in g t h ese var iables, exact ly as with SQL. Th e as
k eywor d m ay be om it t ed. Mor eover , t h e var iable it self can be om it t ed,
an d in t h is case, t h e n am e of t h e collect ion it self ser ves as a var iable
n am e t o r an ge over it.
Exam ple:
select couple(student: Students.name, professor: z.name)
from Students,
Students.takes y,
y.taught_by z
where z.rank = "full professor"
O2 note
In O2 an addit ion al syn t ax is allowed t o declar e a var iable x:
"... f r om x i n collect ion ...".
Th is syn t ax will also be in clu ded in t h e n ext r elease of t h e ODM G
stan dar d.
4.3.8.3 Predicate
In a select -fr om -wh er e qu er y, t h e w here clau se can be om it t ed, wit h t h e
m ean in g of a t r u e pr edicat e.
4.3.9 Group-by Operator
If select_query is a select -fr om -wh er e qu er y, partition_attributes is a
st r u ct u r e expr ession an d pred icate a boolean expr ession , t h en
select_query group by partition_attributes
is an expr ession an d
select_query group by partition_attributes having pred icate
is an expr ession .
Th e car tesian pr odu ct visit ed by t h e select oper at or is split in t o
par t it ion s. For each elem en t of t h e car t esian pr odu ct, t h e par t it ion
at t r ibu t es ar e evalu at ed. All elem en t s wh ich m at ch t h e sam e valu es
accor din g t o t h e given par t it ion at tr ibu t es, belon g t o t h e sam e par tit ion .
Th u s t h e par t it ion ed set , aft er t h e gr ou pin g oper at ion is a set of
st r u ct u r es: each st r u ct u r e h as t h e valu ed pr oper t ies for t h is par t ition
(t h e valu ed partition_attributes), com plet ed by a pr oper t y wh ich is
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con ven tion ally called partition an d wh ich is t h e bag of all object s
m atch in g t h is par t icu lar valu ed par t it ion .
If t h e par t it ion att r ibu t es ar e:
att1: e1, att2: e2, ... , attn: en ,
t h en t h e r esu lt of t h e gr ou pin g is of t ype
set< struct(att1: type_of(e1), att2: type_of(e2),...,
attn: type_of(en),
partition: bag< type_of(grouped elements) >)
Th e type of gr ou ped elem en t s is defin ed as follows.
If t h e from clau se declar es t h e var iables v1 on collect ion col1, v2 on
col2, ..., vn on coln, t h e gr ou ped elem en t s for m a st r u ct u r e wit h on e
at t r ibu t e "vk" for each collect ion h avin g t h e type of th e elem en t s of t h e
cor r espon din g collect ion .
partition: bag< struct(v1: type_of(col1 elements), ... ,
vn: type_of(coln elements))>.
If a collection colk h as n o var iable declar ed t h e cor r espon din g att r ibu t e
h as an in t er n al syst em n am e.
Th is par t it ion ed set m ay t h en be filt er ed by t h e pr edicate of a having
clau se. Fin ally, th e r esu lt is com pu ted by evalu at in g t h e select clau se for
t h is par t it ion ed an d filt er ed set .
Th e having clau se can t h u s apply aggr egat e fu n ct ion s on partition,
lik ewise t h e select clau se can r efer t o partition t o com pu t e t h e fin al
r esu lt . Bot h clau ses can r efer also t o th e par t ition at t r ibu t es.
Exam ple:
select
*
from
group by
Employees e
low:
e.salary < 1000,
medium:
e.salary >= 1000 and salary < 10000,
high:
e.salary >= 10000
Th is gives a set of t h r ee elem en t s, each of wh ich h as a pr oper t y called
partition wh ich con t ain s t h e bag of em ployees t h at en t er in t h is
cat egor y. So t h e t ype of t h e r esu lt is:
set<struct(low: boolean, medium: boolean, high: boolean,
partition: bag<struct(e: Employee)>)>
Th e secon d for m en h an ces th e fir st on e with a having clau se wh ich
en ables you t o filt er t h e r esu lt u sin g aggr egat ive fu n ct ion s wh ich oper at e
on each par t it ion .
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Exam ple:
select
department,
avg_salary: avg(select p.e.salary from partition p)
from Employees e
group by department: e.deptno
having avg(select p.e.salary from partition p) > 30000
Th is gives a set of cou ples: depar t m en t an d aver age of t h e salar ies of t h e
em ployees wor k in g in t h is depar t m en t , wh en t h is aver age is m or e th an
30000. So t h e t ype of t h e r esu lt is:
bag<struct(department: integer, avg_salary: float)>
O2 note
In O2 t h e syn t ax of partition_attributes does n ot accept t h e k eywor d
st r u c t an d t h u s is always given as a list of cr iter ia separ at ed by
com m as. See Sect ion 4.4.1.
4.3.10 Order-by Operator
If select_query is a select -fr om -wh er e or a select -fr om -wh er e-gr ou p_by
qu er y, an d if e1, e2, ..., en ar e expr ession s, t h en
select_query order by e1, e2, ..., en
is an expr ession . It r et u r n s a list of t h e selected elem en t s sor t ed by th e
fu n ct ion e1, an d in side each su bset yieldin g t h e sam e e1, sor t ed by e2,
... , an d t h e fin al su bsu b...set , sor t ed by en.
Exam ple:
select p from Persons p order by p.age, p.name
Th is sor t s t h e set of per son s on t h eir age, t h en on t h eir n am e an d pu t s
t h e sor t ed object s in t o th e r esu lt as a list .
Each sor t expr ession cr it er ion can be followed by t h e k eywor d asc or
d esc, specifyin g r espect ively an ascen din g or descen din g or der . Th e
defau lt or der is t h at of t h e pr eviou s declar at ion . For t h e fir st expr ession ,
t h e defau lt is ascen din g.
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Language Definition : Indexed Collection
Exam ple:
select * from p in Persons
order by p.age desc, p.name asc, p.department
4.3.11
Indexed Collection Expressions
4.3.11.1 Getting the i-th Element of an Indexed Collection
If e1 an d e2 ar e expr ession s, e1 is a list or an ar r ay, e2 is an in t eger , t h en
e1[e2] is an expr ession . Th is ext r act s th e e2+1 th elem en t of t h e
in dexed collect ion e1. Not ice t h at t h e fir st elem en t h as t h e r an k 0.
Exam ple:
list (a,b,c,d) [1]
Th is r et u r n s b.
Exam ple:
element (select x
from Courses x
where x.name = "math" and
x.number ="101").requires[2]
Th is r et u r n s t h e t h ir d pr er equ isite of M at h 101.
4.3.11.2 Extracting a Subcollection of an Indexed Collection.
If e1, e2, an d e3 ar e expr ession s, e1 is a list or an ar r ay, e2 an d e3 ar e
in t eger s, th en e1[e2:e3] is an expr ession . Th is ext r act s th e
su bcollect ion of e1 st ar t in g at posit ion e2 an d en din g at posit ion e3.
Exam ple:
list (a,b,c,d) [1:3]
Th is r et u r n s list (b,c,d).
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Exam ple:
element (select x
from Courses x
where x.name="math" and
x.number="101").requires[0:2]
Th is r et u r n s t h e list con sist in g of t h e fir st t h r ee pr er equ isit es of Mat h
101.
4.3.11.3 Getting the First and Last Elements of an Indexed Collection
If e is an expr ession , if <op> is an oper at or fr om {first, last}, e is a
list or an ar r ay, th en <op>(e) is an expr ession . Th is extr acts t h e fir st an d
last elem en t of a collection .
Exam ple:
first(element(select x
from Courses x
where x.name="math" and
x.number="101").requires)
Th is r et u r n s t h e fir st pr er equ isit e of Mat h 101.
4.3.11.4 Concatenating Two Indexed Collections
If e1 an d e2 ar e expr ession s, if e1 an d e2 ar e bot h lists or both ar r ays,
t h en e1+e2 is an expr ession . Th is com pu t es th e con cat en at ion of e1 an d
e 2.
Exam ple:
list (1,2) + list( 2,3)
Th is qu er y gen er at es list (1,2,2,3).
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Language Definition : Binary Set Expressions
4.3.12
Binary Set Expressions
4.3.12.1
Union, Intersection, Difference
If e1 an d e2 ar e expr ession s, if <op> is an oper at or fr om {union,
except, intersect}, if e1 an d e2 ar e set s or bags, t h en e1 <op> e2
is an expr ession . Th is com pu t es set t h eor et ic oper ation s, u n ion ,
differ en ce, an d in t er sect ion on e1 an d e2, as defin ed in Ch apt er 2.
Wh en th e collection k in ds of t h e oper an ds ar e differ en t (bag an d set ), t h e
set is con ver t ed in t o a bag befor eh an d an d t h e r esu lt is a bag.
Exam ples:
Student except Ta
Th is r et u r n s t h e set of st u den ts wh o ar e n ot Teach in g Assist an t s.
bag(2,2,3,3,3) union bag(2,3,3,3)
Th is bag expr ession r etu r n s bag(2,2,3,3,3,2,3,3,3)
bag(2,2,3,3) intersect bag(2,3,3,3)
Th e in ter sect ion of 2 bags yields a bag th at con t ain s th e m in im u m for
each of th e m u lt iply valu es. So t h e r esu lt is: bag(2,3,3)
bag(2,2,3,3,3) except bag(2,3,3,3)
Th is bag expr ession r etu r n s bag(2)
4.3.12.2 Inclusion
If e1 an d e2 ar e expr ession s wh ich den ote set s or bags, if <op> is an
oper ator fr om
{<, <=, >, >=}, th en e1 <op> e2 is an expr ession wh ose
valu e is a boolean .
Wh en t h e oper an ds ar e differ en t k in ds of collect ion s (bag an d set ), t h e
set is fir st con ver t ed in t o a bag.
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e1 < e2 is t r u e if e1 is in clu ded in t o e2 bu t n ot equ al t o e2
e1 <= e2 is t r u e if e1 is in clu ded in t o e2
Exam ple:
set(1,2,3) < set(3,4,2,1)
is t r u e.
4.3.13
Conversion Expressions
4.3.13.1 Extracting the Element of a Singleton
If e is a collect ion -valu ed expr ession , element(e) is an expr ession . Th is
t ak es t h e sin glet on e an d r et u r n s it s elem en t . If e is n ot a sin glet on t h is
r aises an exception .
Exam ple:
element(select x from Professors x
where x.name ="Turing")
Th is r et u r n s t h e pr ofessor wh ose n am e is Turing (if t h er e is on ly on e).
4.3.13.2 Turning a List into a Set
If e is a list expr ession , listtoset(e) is an expr ession . Th is con ver ts
t h e list in t o a set, by for m in g th e set con t ain in g all t h e elem en t s of t h e
list .
Exam ple:
listtoset (list(1,2,3,2))
Th is r et u r n s t h e set con t ain in g 1, 2, an d 3.
O2 note
To car r y ou t t h e r ever se oper at ion (set t o list ) you u se th e or der by
oper at or . If you ar e n ot in t er est ed in a given or der you can u se "*" as
sh own in t h e followin g qu er y:
select e from e in aSet order by *
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4.3.13.3 Removing Duplicates
If e is an expr ession wh ose valu e is a collect ion , t h en
distinct(e)
is an expr ession wh ose valu e is th e sam e collect ion aft er r em ovin g t h e
du plicat ed elem en t s. If e is a bag, distinct(e) is a set . If e is an or der ed
collection , t h e r elat ive or der in g of t h e r em ain in g elem en t s is pr eser ved.
4.3.13.4 Flattening Collection of Collections
If e is a collect ion -valu ed expr ession , flatten(e) is an expr ession . Th is
con ver t s a collect ion of collect ion s of t in t o a collect ion of t. So th is
flat t en in g oper at es at t h e fir st level on ly.
Assu m in g t h e type of e to be col1<col2<t>>,
t h e r esu lt of flatten(e) is:
• If col2 is a set (r esp. a bag), th e u n ion of all col2<t> is don e an d th e
r esu lt is a set<t> (r esp. bag<t>)
• If col2 is a list (r esp. an ar r ay) an d col1 is a list (r esp. an ar r ay) as
well, t h e con cat en at ion of all col2<t> is don e followin g t h e or der in
col1 an d t h e r esu lt is col2<t>, wh ich is t h u s a list (r esp. an ar r ay).
Of cou r se du plicates, if an y, ar e m ain t ain ed by t h is oper ation .
• If col2 is a list or an ar r ay an d col1 is a set or a bag, t h e list s or
ar r ays ar e con ver t ed in to set s, t h e u n ion of all t h ese set s is don e an d
th e r esu lt is a set<t>, t h er efor e with ou t du plicat es.
Exam ples:
flatten(list(set(1,2,3), set(3,4,5,6), set(7)))
Th is r et u r n s t h e set con t ain in g 1,2,3,4,5,6,7.
flatten(list(list(1,2), list(1,2,3)))
Th is r et u r n s list(1,2,1,2,3).
flatten(set(list(1,2), list(1,2,3)))
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Th is r et u r n s t h e set con t ain in g 1,2,3.
4.3.13.5 Typing an Expression
If e is an expr ession , if c is a t ype n am e, t h en (c)e is an expr ession . Th is
asser ts t h at e is an object of class t ype c.
If it tu r n s ou t t h at it is n ot t r u e, an except ion is r aised at r u n t im e. Th is
is u sefu l to access a pr oper t y of an object wh ich is st at ically k n own t o be
of a su per class of t h e specified class.
Exam ple:
select ((Employee) s).salary
from Students s
where s in (select sec.assistant from Sections sec)
Th is r et u r n s t h e set of salar ies of all stu den t s wh o ar e teach in g
assist an t s, assu m in g th at Students an d Sections ar e th e exten ts of t h e
classes Student an d Section.
4.3.14
Function Call
If f is a fu n ct ion n am e, if e1 , e2 , ..., en ar e expr ession s, t h en
f() and
f(e1, e2, ... , en)
ar e expr ession s wh ose valu e is th e valu e r et u r n ed by t h e fu n ct ion , or t h e
object nil, wh en t h e fu n ct ion does n ot r etu r n an y valu e. Th e fir st for m
allows you to call a fu n ct ion wit h ou t a par am et er , wh ile t h e secon d on e
calls a fu n ct ion wit h t h e par am et er s e1, e2, ..., en.
OQL does n ot defin e in wh ich lan gu age t h e body of su ch a fu n ct ion is
wr it t en . Th is feat u r e allows you t o sm ooth ly ext en d t h e fu n ct ion alit y of
OQL wit h ou t ch an gin g t h e lan gu age.
4.3.15
Scope Rules
Th e from par t of a select -fr om -wh er e qu er y in t r odu ces explicit or im plicit
var iables t o r an ge over t h e filt er ed collection s. An exam ple of an explicit
var iable is:
select ... from Persons p ...
wh ile an im plicit declar at ion wou ld be:
select ... from Persons ...
Th e scope of t h ese var iables r each es all par t s of t h e select -fr om -wh er e
expr ession in clu din g n est ed su b-expr ession s.
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Language Definition : Scope Rules
Th e group by par t of a select-fr om -wh er e-gr ou p_by qu er y in t r odu ces t h e
n am e partition alon g wit h possible explicit at t r ibu t e n am es wh ich
ch ar act er ize t h e par t ition . Th ese n am es ar e visible in t h e cor r espon din g
having an d select par ts, in clu din g n ested su b-expr ession s wit h in th ese
par ts.
In side a scope, you u se th ese var iable n am es t o con st r u ct path
expr ession s an d r each pr oper t ies (at t r ibu t es an d oper at ion s) wh en t h ese
var iables den ot e com plex object s. For in st an ce, in t h e scope of t h e fir st
fr om clau se above, you access th e age of a per son by p.age.
Wh en t h e var iable is im plicit , as in t h e secon d fr om clau se, you u se t h e
n am e of t h e collection dir ect ly, Persons.age.
However , wh en t h er e is n o am bigu it y, you can u se t h e pr oper ty n am e
dir ect ly as a sh or tcu t , wit h ou t u sin g t h e var iable n am e t o open t h e scope
(t h is is m ade im plicit ly), wr it in g sim ply: age. Th er e is n o am bigu ity wh en
a pr oper t y n am e is defin ed for on e an d on ly on e object den ot ed by a
visible var iable.
To su m m ar ize, a n am e appear in g in a (n est ed) qu er y is look ed u p in th e
followin g or der :
• a var iable in th e cu r r en t scope, or
• a n am ed qu er y in t r odu ced by t h e d efine clau se, or
• a n am ed object , i.e., an en t r y poin t in t h e database, or
• an at tr ibu t e n am e or an oper at ion n am e of a var iable in t h e cu r r en t
scope, wh en t h er e is n o am bigu it y, i.e., t h is pr oper t y n am e belon gs t o
on ly on e var iable in t h e scope.
Exam ple:
Assu m in g t h at in th e cu r r en t sch em a t h e n am es Per son s an d Cit ies ar e
defin ed.
select
scop e1
from Persons,
Cities c
where exists(select scop e2 from children as child)
or count (select scop e3 , (select scop e4 from
partition)
from
children p,
scop e5 v
group by age: scop e6
)
In scope1 , we see t h e n am es: Per son s, c, Cit ies, all pr oper t y n am es of
class Per son an d class Cit y as soon as t h ey ar e n ot pr esen t in bot h
classes, an d t h ey ar e n ot called "Per son s", "c", n or "Cit ies".
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In scope2 , we see t h e n am es: ch ild, Per son s, c, Cit ies, t h e pr oper t y
n am es of th e class Cit y wh ich ar e n ot pr oper t y of t h e class Per son . No
at t r ibu t e of t h e class Per son can be accessed dir ect ly sin ce th ey ar e
am bigu ou s bet ween "ch ild" an d "Per son s".
In scope3 , we see t h e n am es: age, par t it ion , an d t h e sam e n am es fr om
scope1, except "age" an d "par t ition ", if th ey exist.
In scope4 , we see t h e n am es: age, par t it ion , p, v, an d t h e sam e n am es
fr om scope1, except "age", "par t ition ", "p" an d "v", if t h ey exist .
In scope5 , we see t h e n am es: p, an d t h e sam e n am es fr om scope1, except
"p", if it exist s.
In scope6 , we see t h e n am es: p, v, Per son s, c, Cit ies, t h e pr oper t y n am es
of t h e class Cit y wh ich ar e n ot pr oper t y of t h e class Per son . No att r ibu t e
of t h e class Per son can be accessed dir ectly sin ce t h ey ar e am bigu ou s
between "ch ild" an d "Per son s".
O2 note
Im plicit at t r ibu t e scope is n ot available wit h O2 . You m u st always access
an at t r ibu t e with t h e dot n otation : v.att.
4.4 Syntactical Abbreviations
OQL defin es an or t h ogon al expr ession lan gu age, in t h e sen se t h at all
oper at or s can be com posed with each ot h er s as soon as t h e types of t h e
oper an ds ar e cor r ect . To ach ieve t h is pr oper t y, we h ave defin ed a
fu n ct ion al lan gu age with sim ple (lik e +) or com posit e oper at or s (lik e
select fr om wh er e gr ou p_by or der _by) wh ich always deliver a r esu lt in t h e
sam e t ype syst em an d wh ich t h u s can be r ecu r sively oper at ed wit h ot h er
oper at ion s in t h e sam e qu er y.
In or der t o accept th e wh ole DM L qu er y par t of SQL, as a valid syn t ax for
OQL, OQL is added som e ad-h oc con st r u ct ion s each t im e SQL in t r odu ces
a syn t ax wh ich can n ot en t er in t h e cat egor y of t r u e oper at or s. Th is
sect ion gives t h e list of t h ese con st r u ct ion s t h at we call "abbr eviat ion s",
sin ce t h ey ar e com plet ely equ ivalen t t o a fu n ction al OQL expr ession
wh ich is also given . Doin g t h at , we t h u s give at t h e sam e tim e t h e
sem an t ics of t h ese con st r u ct ion s, sin ce all oper ator s u sed for t h is
descr iption h ave alr eady been defin ed.
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4.4.1
Structure Construction
Th e st r u ct u r e con st r u ctor was in t r odu ced in Sect ion 4.3.4.2. Alt er n at e
syn t ax ar e allowed in t wo con text s: select clau se an d gr ou p-by clau se.
In both con t exts, t h e SQL syn t ax is accept ed, alon g wit h t h e on e alr eady
defin ed.
sel ec t projection {, projection } ...
select ... gr ou p by projection {, projection }
wh er e projection is in on e of t h e followin g for m s:
• (i) expr ession as iden t ifier
• (ii) iden t ifier : expr ession
• (iii) expr ession
Th is is an alt er n ate syn t ax for :
st r u c t (iden t ifier : expr ession {, iden t ifier : expr ession })
If t h er e is on ly on e projection an d t h e syn t ax (iii) is u sed in a select clau se,
t h en it is n ot in t er pr et ed as a st r u ct u r e con st r u ction bu t r at h er t h e
expr ession st an ds as it is. Fu r t h er m or e, a (iii) expr ession is on ly valid if
it is possible t o in fer t h e n am e of t h e cor r espon din g at t r ibu t e (t h e
iden t ifier ). Th is r equ ir es t h at t h e expr ession den otes a path expr ession
(possibly of len gt h on e) en din g in a pr oper t y wh ose n am e is th en ch osen
as t h e iden t ifier .
Exam ple:
select p.name,
salary,
from Professors p,
student_id
p.teaches
Th is qu er y r et u r n s a bag of st r u ct u r es:
bag<struct(name: string, salary: float, student_id:
integer)>
O2 note
O2 accept s t h e 3 alt er n atives of t h e projection syn t ax in th e select par t ,
as well as t h e struct syn t ax. In t h e group by par t , O 2 accepts t h e 3
alt er n at ives bu t does n ot accept t h e struct syn t ax.
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4.4.2
Aggregate Operators
Th ese oper at or s wer e in t r odu ced in Sect ion 4.3.7.4. SQL adopt s a
n ot at ion wh ich is n ot fu n ction n al for t h em . So OQL accept s t h is syn t ax
t oo.
If we defin e aggregate as on e of m i n , m ax , cou n t , su m an d av g,
select count(*) from ...
is equ ivalen t t o: count(select * from ...)
select aggregate(query) from ...
is equ ivalen t t o: aggregate(select query from ...)
select aggregate(distinct query) from ...
is equ ivalen t t o: aggregate(distinct( select query from ...)
O2 note
O2 does n ot su ppor t Aggr egat e Oper at or abbr eviat ion s.
4.4.3
Composite Predicates
If e1 an d e2 ar e expr ession s, e2 is a collect ion , e1 h as t h e t ype of it s
elem en t s, if relation
is a r elat ion al oper at or (=, !=, <, <=, > , >=), th en
e1 relation some e2
an d
e1 relation any e2 an d e1 relation all e2
ar e expr ession s wh ose valu e is a boolean .
Th e t wo fir st pr edicat es ar e equ ivalen t t o:
exists x in e2: e1 relation x
Th e last pr edicat e is equ ivalen t t o:
for all x in e2: e1 relation x
Exam ple:
10 < some (8,15, 7, 22)
is t r u e
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OQL BNF : String Literal
O2 note
In O2 Com posite Pr edicat e abbr eviat ion s ar e n ot su ppor t ed.
4.4.4
String Literal
OQL accept s sim ple qu ot es as well t o delim it a st r in g (see Sect ion
4.3.3.1), as SQL does. Th is in t r odu ces an am bigu it y for a st r in g wit h on e
ch ar act er wh ich t h en h as t h e sam e syn t ax as a ch ar act er lit er al. Th is
am bigu it y is solved by con t ext .
O2 note
In O2 a st r in g m u st be delim it ed by dou ble qu ot es.
4.5 OQL BNF
Th e OQL gr am m ar is given u sin g a BNF-lik e n ot at ion .
• { sym bol } m ean s a sequ en ce of 0 or m or e sym bol(s).
• [sym bol ] m ean s an opt ion al sym bol. Do n ot con fu se wit h t h e
separ at or s [ ]
• keyword is a t er m in al of t h e gr am m ar . Keywor ds ar e n ot case
sen sitive.
• xxx_n am e h as t h e syn t ax of an iden t ifier
• xxx_lit er al is self explan at or y, e.g., "a st r in g" is a st r in g_lit er al
• bin d_ar gu m en t stan ds for a par am et er wh en em bedded in a
pr ogr am m in g lan gu age, e.g., $3i.
Th e n on t er m in al query st an ds for a valid qu er y expr ession . Th e
gr am m ar is pr esen t ed as r ecu r sive r u les pr odu cin g valid qu er ies. Th is
explain s wh y m ost of t h e t im e t h is n on t er m in al appear s on t h e left side
of ::=. Of cou r se, all oper at or s expect t h eir "qu er y" oper an ds t o be of t h e
r igh t t ype. Type con st r ain t s wer e discu ssed in t h e pr eviou s sect ion s.
Th ese r u les m u st be com pleted by t h e pr ior it y of OQL oper at or s wh ich is
given aft er t h e gr am m ar . Som e syn t act ical am bigu it ies ar e solved
sem an t ically fr om t h e t ypes of t h e oper an ds.
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4.5.1
Grammar
4.5.1.1 Axiom (see Sections 4.3.1, 4.3.2)
query_program ::={define_query;} query
define_query ::=define identifier as query
4.5.1.2 Basic (see Section 4.3.3)
query ::= nil
query ::= true
query ::= false
query ::= integer_literal
query ::= float_literal
query ::= character_literal
query ::= string_literal
query ::= entry_name
query ::= query_name
query ::= bind_argument1
query ::= from_variable_name
query ::= (query)
4.5.1.3 Simple Expression (see Section 4.3.5)
query ::= query + query2
query ::= query - query
query ::= query * query
query ::= query / query
query ::= - query
query ::= query mod query
query ::= abs (query)
query ::= query || query
4.5.1.4 Comparison (see Section 4.3.5)
query ::= query comparison_operator query
query ::= query like string_literal
comparison_operator ::= =
comparison_operator ::= !=
comparison_operator ::= >
comparison_operator ::= <
comparison_operator ::= >=
1. A bin d ar gu m en t allows to bin d expr ession s fr om a pr ogr am m in g lan gu age to a qu er y
wh en em bedded in to th is lan gu age (see Ch apters on lan gu age bin din gs).
2. Th e oper at or + is also u sed for list an d ar ray con cat en at ion .
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comparison_operator ::= <=
4.5.1.5 Boolean Expression (see Section 4.3.5)
query ::= not query
query ::= query and query
query ::= query or query
4.5.1.6 Constructor (see Section 4.3.4)
query ::= type_name ([query] )
query ::= type_name (identifier:query {, identifier: query})
query ::= struct (identifier: query {, identifier: query})
query ::= set ([query {, query}])
query ::= bag ([query {,query}])
query ::= list ([query {,query}])
query ::= (query, query {, query})
query ::=[ list](query .. query)
query ::= array ([query {,query}])
4.5.1.7 Accessor (see Sections 4.3.6, 4.3.11, 4.3.14, 4.3.15)
query ::= query dot attribute_name
query ::= query dot relationship_name
query ::= query dot operation_name
query ::= query dot operation_name( query {,query} )
dot
::= . | ->
query ::= * query
query ::= query [query]
query ::= query [query:query]
query ::= first (query)
query ::= last (query)
query ::= function_name( [ query {,query} ] )
4.5.1.8
Collection Expression (see Sections 4.3.7, 4.4.3)
query ::= for all identifier in query: query
query ::= exists identifier in query: query
query ::= exists(query)
query ::= unique(query)
query ::= query in query
query ::= query comparison_operator quantifier query
quantifier ::=
some
quantifier ::=
any
quantifier ::=
all
query ::= count (query)
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4
query ::= count (*)
query ::= sum (query)
query ::= min (query)
query ::= max (query)
query ::= avg (query)
4.5.1.9 Select Expression (see Sections 4.3.8, 4.3.9, 4.3.10)
query ::= select [ distinct ] projection_attributes
from variable_declaration {, variable_declaration}
[where query]
[group by partition_attributes]
[having query]
[order by sort_criterion {, sort_criterion}]
projection_attributes ::= projection {, projection}
projection_attributes ::= *
projection ::= query
projection ::= identifier: query
projection ::= query as identifier
variable_declaration ::= query [ [ as ] identifier]
partition_attributes ::= projection {, projection}
sort_criterion ::= query [ordering]
ordering ::= asc
ordering ::= desc
4.5.1.10
Set Expression (see Section 4.3.12)
query ::= query intersect query
query ::= query union query
query ::= query except query
4.5.1.11
Conversion (see Section 4.3.13)
query ::= listtoset (query)
query ::= element (query)
query ::= distinct(e)
query ::= flatten (query)
query ::= (class_name) query
4.5.2
Operator Priorities
Th e followin g oper at or s ar e sor t ed by decr easin g pr ior it y. Oper at or s on
t h e sam e lin e h ave t h e sam e pr ior it y an d gr ou p left -t o-r igh t .
()
88
[]
. ->
ODM G OQL User Man u al
OQL BNF : Operator Priorities
n ot
- (u n ar y)
+ (u n ar y)
in
*
/
m od
i n t er sect
+
-
u n i on
ex cep t
< > <=
oper ator s)
=
!=
an d
>=
||
< som e < an y < al l
(et c ... for all com par ison
lik e
ex i st s
for all
or
..
:
,
(iden t ifier ) th is is t h e cast oper at or
order
having
group by
where
from
select
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4
90
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IN DEX
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INDEX
Sym bol s
C
+ 36
C 11
C++
In t er face 11
A
Accessor 87
Addit ion of set s 36
Aggr egat ive oper at or s 54
Ar ch itect u r e
O2 10
Ar it h m etic 86
Ar r ay 21, 22, 25
Con st r u ctin g 63
Set con ver sion 37
array 27, 29, 51
Ar r ay valu e 21
Atom ic valu e 20
Att r ibu t e 66
avg 30, 54, 70
C++ bin din g 44, 45
Class in dicat or 54
Collection 22, 44, 49, 79
in dexed expr ession 75
Nam ed 39
Collection expr ession 87
Com bin in g oper at or s 28, 38
Com par ison 86
con cat en ation 76
Con st r u ction
Ar r ay 51
Bag 51
List 51
Set 51
Str u ct 51
Con st r u ctor 27, 51, 87
Con ver sion 37, 78, 88
count 30, 54, 70
Cr eat in g object s 29
B
D
Bag 22, 29
Con st r u ctin g 63
Data m an ipu lat ion 51
Database en t r y poin t 20
define 30, 36, 54
differ en ce 77
distinct 23, 49
bag 27, 29, 51
Boolean 87
Br owser In t er face 12
Un ix 12
Win dows NT 13
by 35
92
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INDEX
E
I
element 31, 78
except 36, 54, 77
Exist en t ial qu an t ificat ion 38, 54, 69
exists 31, 54
intersect 36, 54, 77
in t er sect ion 77
J
F
first 76
flatten 38
Flatt en in g 79
forall ... in 54
from 50
G
group ... by 32, 54, 55, 72
H
H yper t ext lin k s 13
J ava 11
J ava bin din g 44
J oin 50
J oin qu er y 24
L
last 76
Lat e bin din g 53
like 35
List 21, 22, 25
Con st r u ct in g 62
Set con ver sion 37
Valu es 21
list 27, 51, 78
listtoset 37
M
max 30, 54, 70
Mem ber sh ip 69
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INDEX
M et h od call 22, 52
M et h od in vok in g 52
min 30, 54, 70
M ot if 13
N
name 31
Nam ed
Collection 39
Object s 20
Qu er y 30
Valu es 20
O
O2
Ar ch it ectu r e 10
O 2 C 11
O 2 Cor ba 11
O 2 DBAccess 11
O 2 En gin e 10
O 2 Gr aph 11
O 2 Kit 11
O 2 Look 11, 12
O 2 OD BC 11
O 2 Stor e 10
O 2 Tools 11
O 2 Web 11
Object
Cr eat ion 29
Nam ed 20
Oper at ion 67
Oper at or 30, 54
- 36
* 36
+ 36
Aggr egat ive 54
avg 30, 54, 70
Com bin in g 28, 38
Com position 54
count 30, 54, 70
define 30, 54
element 31
except 36, 54
exists 31, 54
flatten 38
forall...in 54
group...by 32, 54, 55, 72
intersect 36, 54
like 35
max 30, 54, 70
min 30, 54, 70
order by 35, 74
Set 36, 38, 54
sum 30, 54, 70
union 36, 54
Wild-car d 38
OQL 11, 12, 17
Oper at or s 30
Rat ion al 43
Resu lt 27
or der by 35, 54, 74
P
partition 33
Pat h expr ession s 24, 49
Polym or ph ism 53
Pr edicat e 50
Object s 61
ODMG m odel 44
ODMG stan dar d 44, 57
94
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INDEX
Q
T
Qu er y
Basic 18, 86
Nam ed 30
Resu lt 27, 30
Test in g on n il 25
Typin g 80
U
R
Ref 44
Relation sh ip 44, 49, 66
S
select 55
Select fr om wh er e 70
select from where 22, 49
Set 22, 22
Con st r u ctin g 62
List con ver sion 37
Oper at or s 36, 54
set 27, 29, 51, 77, 78
Set expr ession 88
struct 27, 29, 29, 51
St r u ct valu e 21
st r u ct u r e 62
Su bcollect ion 75, 75
sum 30, 54, 70
Syst em
Ar ch it ectu r e 10
union 36, 54, 77
Un iver sal qu an t ificat ion 38, 54, 68
Un ix 12
V
Valu e
Ar r ay 21
At om ic 20
List 21
Nam ed 20
Str u ct 21
W
where 50
Win dows NT 13
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