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ODMG OQL User Manual

1. ODMG OQL User Manual Release 5 0 February 1998 2 Information in this document is subject to change without notice and should not be construed as a commitment by O Technology The software described in this document is delivered under a license or nondisclosure agreement The software can only be used or copied in accordance with the terms of the agreement It is against the law to copy this software to magnetic tape disk or any other medium for any purpose other than the purchaser s own use Copyright 1992 1998 O Technology All rights reserved No part of this publication can be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopy without prior written permission of Op Technology Oo OoEngine API O2C O2DBAccess OsEngine OoGraph OoKit OsLook OsStore OsTools and O2Web are registered trademarks of Op Technology SQL and AIX are registered trademarks of International Business Machines Corporation Sun SunOS and SOLARIS are registered trademarks of Sun Microsystems Inc X Window System is a registered trademark of the Massachusetts Institute of Technology Unix is a registered trademark of Unix System Laboratories Inc HPUX is a registered trademark of Hewlett Packard Company BOSX is a registered trademark of Bull S A IRIX is a registered trademark of Siemens Nixdorf A G NeXTStep is a registered tradem
2. Experience of SQL though not a prerequisite will facilitate the OQL learning process 1 The Object Database Standard ODMG 93 release 1 2 Edited by R G G Cattell 1996 Morgan Kaufman Publishers ODMG OQL User Manual 2 1 GETTING STARTED 2 1 Basic queries All the examples shown below are based on the following Os schema In OC 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 2 2 ODMG OQL User Manual Basic queries In C class Company public d_String name d_Set lt d_Ref lt Employee gt gt employees d_List lt d_Ref lt Client gt gt clients char title return name class item d String what double price class Client public d String name d_Array lt item gt order class Employee public d_String name d_Date birthday d_String position float salary int age Two persistent roots are also defined An object Globe and a collection the_employees
3. To summarize a name appearing in a nested query is looked up in the following order a variable in the current scope or anamed query introduced by the define clause or e a named object i e an entry point in the database or e an attribute name or an operation name of a variable in the current scope when there is no ambiguity i e this property name belongs to only one variable in the scope Example Assuming that in the current schema the names Persons and Cities are defined select scope from Persons Cities c where exists select scope2 from children as child or count select scope3 select scope4 from partition from children p scoped v group by age scope6 In scopet we see the names Persons c Cities all property names of class Person and class City as soon as they are not present in both classes and they are not called Persons c nor Cities In scope2 we see the names child Persons c Cities the property names of the class City which are not property of the class Person No attribute of the class Person can be accessed directly since they are ambiguous between child and Persons ODMG OQL User Manual 4 27 In scope3 we see the names age partition and the same names from scopel except age and partition if they exist In scope4 we see the names age partition p v and the same names from scopel except age partition p
4. 3 an object query language for this model OQL 4 a C binding allowing C programs to operate on a database compliant to the object model 5 a Smalltalk binding allowing Smalltalk programs to operate on a database compliant to the object model 3 2 ODMG OQL User Manual The ODMG model 3 2 The ODMG model The ODMG object model supports the notion of classes of objects with attributes and methods of inheritance and specialization It offers the classical types to deal with string date time time interval and timestamp And finally it supports the notions of relationships and collections ODMG 93 introduces a set of predefined generic collection classes Set lt T gt Bag lt T gt a multi set i e a set with repeated elements Varray T a variable size array List lt T gt a variable size and insertable array An object refers to another object through a Ref A Ref behaves as a C pointer but with more semantics it is a persistent pointer but referential integrity can be expressed in the schema and maintained by the system This is done by declaring the relationship as symmetric Combining relationships and collections an object can relate to more than one object through a relationship Therefore 1 1 relationships 1 n relationships and n m relationships can be supported with the same guarantee of referential integrity ODMG 93 enables explicit names to be given to any object or collection
5. 4 3 3 2 Named Objects If eis a named object then e is an expression It defines the entity attached to the name Example Students This query defines the set of students We have assumed here that the name Students exists which corresponds to the extent of objects of the class Student 4 3 3 3 Iterator Variable If xis a variable declared in a from part of a select from where then x is an expression whose value is the current element of the iteration over the corresponding collection 4 3 3 4 Named Query If define q as eis a query definition expression then qis an expression Example This query returns the student with name Doe It refers to the query definition expression declared in Section 4 3 2 ODMG OQL User Manual 4 5 4 3 4 Construction Expressions 4 3 4 4 Constructing Objects If t is a type name Pi po Pn are properties of t and ei eo are expressions then t py e1 Pa e is an expression This defines a new object of type t whose properties p po 1Pp are initialized with the expressions e eg e Thetypeof e must be compatible with thetype of p If t is atype name of a collection and eis a collection literal then t e is a collection object The type of e must be compatible with t Examples Employee name Peter boss Chairman This creates a mutable Employee object vectint set 1 3 10 This creates a mutable set object
6. 2 3 3 bag 2 2 3 3 3 except bag 2 3 3 3 This bag expression returns bag 2 Inclusion If e and e are expressions which denote sets or bags if op is an operator from lt lt gt gt then e op ej is an expression whose value is a boolean When the operands are different kinds of collections bag and set the set is first converted into a bag ei lt ejistrueif e is included into ea but not equal to e e lt ejis trueif e is included into e ODMG OQL User Manual 4 23 4 3 13 4 3 13 1 4 3 13 2 Example set 1 2 3 lt set 3 4 2 1 is true Conversion Expressions Extracting the Element of a Singleton If e is a collection valued expression element e is an expression This takes the singleton e and returns its element If eis not a singleton this raises an exception Example element select x from Professors x where x name Turing This returns the professor whose nameis Turing if there is only one Turning a List into a Set If e is alist expression listtoset e is an expression This converts the list into a set by forming the set containing all the elements of the list Example listtoset list 1 2 3 2 This returns the set containing 1 2 and 3 Os note To carry out the reverse operation set to list you use the order by operator If you are not interested in a given order you can use shown in
7. Depending on the kind of person of the current p the right incarnation is called Class indicator To go down theclass hierarchy a user may explicitly declare the class of an object that cannot be inferred statically The interpreter then has to check at runtime that this object actually belongs to the indicated class or one of its subclasses For example assuming we know that only students spend their time in following a course of study we can select those persons and get their grade We explicitly indicate in the query that these persons are students select Student p grade from p in Persons where course of study in p activities Operator composition OQL is a purely functional language all operators can be composed freely as long as the type system is respected This is why the language is so simple and its manual so short This philosophy is different from SQL which is an ad hoc language whose composition rules are not orthogonal to the type system Adopting a complete orthogonality makes the language easier to learn without losing the SQL style for simple queries Among the operators offered by OQL but not yet introduced we can mention the set operators union intersect except the universal forall and existential quantifiers exists the order by and group by operators and the aggregative operators count sum min max and avg To illustrate this free composition of operators let us write a rather
8. and v if they exist In scope5 we see the names p and the same names from scopet except p if it exists In scope6 we see the names p v Persons c Cities the property names of the class City which are not property of the class Person No attribute of the class Person can be accessed directly since they are ambiguous between child and Persons O gt note Implicit attribute scope is not available with Os You must always access an attribute with the dot notation v att 4 4 Syntactical Abbreviations OQL defines an orthogonal expression language in the sense that all operators can be composed with each others as soon as the types of the operands are correct To achieve this property we have defined a functional language with simple like or composite operators like select from where group by order by which always deliver a result in the sametype system and which thus can berecursively operated with other operations in the same query In order to accept the whole DML query part of SQL as a valid syntax for OQL OQL is added some ad hoc constructions each time SQL introduces a syntax which cannot enter in the category of true operators This section gives the list of these constructions that we call abbreviations since they are completely equivalent to a functional OQL expression which is also given Doing that wethus give at the same time the semantics of these constructions since all operators used fo
9. elaborate query We want to know the name of the street where the set of employees living on that street and have the smallest average salary compared to the sets of employees living in other streets We proceed step by step and use the define OQL instruction to evaluate temporary results 3 12 ODMG OQL User Manual OQL by example Operator composition 1 Build the extent of class Employee not supported directly by the schema define Employees as select Employee p from p in Persons where has a job in p activities 2 Group the employees by street and compute the average salary in each street 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 The group by operator splits the employees into partitions according to the criterion the name of the street where this person lives The select clause computes in each partition the average of the salaries of the employees belonging to this partition The result of the query is of type Bag lt struct String street float average salary 3 Sort this set by salary define sorted salary map as select s from s in salary map order by s average salary The result is of type List lt struct String street float average salary ODMG OQL User Manual 3 13 OQL RATIONALE 4 Now get the smalle
10. type your query and end with D To leave the query session type D or quit You are now back in the command interpreter and you see the message type your command and end with D You can also use OQL in the O5Tools programming environment Refer to the OsTools User Manual Note In a Windows environment Z Control Z is used instead of D Control D ODMG OQL User Manual 1 7 INTRODUCTION Embedded OQL 1 3 Any valid query can be passed from O5C code to OQL using the system supplied function o2query This is detailed in the OC Reference manual Similarly you can pass a query to a C C Smalltalk or Java program Refer to the respective manuals for details Finally an OQL function exists in OgEngine and is described in the OsEngine API Reference Manual Manual overview This manual is divided up into the following chapters Chapter 1 Introduction This chapter introduces the Op system and the OQL query language It outlines the concepts of the ad hoc query language that allows you to browse the database quickly and efficiently to get the information you want and the embedded query language that you can call from inside your programs Chapter 2 OQL Getting started This chapter introduces the OQL language so you can start to use OQL in order to obtain the exact information you want from your database It describes and illustrates basic and select from where quer
11. 12 4 3 5 0 Lierefarencimg an ODIBOl uii e etia detpb iocket indio dal 4 14 843 7 Collections EXpressions cinnara MM Eb tuM eM MdUE 4 14 4 3 7 4 Universal Quant fication ces iore ec Ee ee eeu 4 14 4 3 7 2 Existential Quantificalioni eerte etie eene nts 4 14 LALNBER Membership Testing ERO 4 15 4 3 14 Aggrecate ACCS T 4 15 CAE E educi di NE adalat orients ctaiat 4 16 2 3 8 L POCO dbi bp NADIE TUH HE OM MIR pM MM NE 4 16 4 3 0 2 Trecator Variables Lou oiii Fori pio HEX UP EOM Spe EEbRM PER ORE 4 17 LAT Kl ooo 4 18 poem Gronn EN os METER 4 18 23 Odder DDSEBOE asse RU pH UM OROEVeR PRA IMPE EDAM NAT P RAE 4 20 43 1l Indexed Collection EXpIOSSIODE odisse sueta ine Pae Ren EEP C tab Ro Red uuES 4 21 4 3 11 1 Getting the i th Element of an Indexed Collection 4 21 4 3 11 2 Extracting a Subcollection of an Indexed Collection 4 21 4 3 11 3Gettingthe First and Last Elements of an Indexed Collection i 4 22 iv ODMG OQL User Manual TABLE OF CONTENTS 4 4 4 5 4 3 11 4 Concatenating Two Indexed Collections 4 22 4 3 12 Viney Set Expressions iesu ee HA p Qe tiM naiiai 4 22 4 3 12 1 Union Intersection Difference eese 4 23 a Mie NI e NERONE 4 23 EXIT LODVSDHIOB Expressi qa ape e Rein su PET E 4 24 4 3 13 1 Extracting the Element of a Singleton 4 24 22132 Turning a List into
12. 2 6 3 10 ODMG OQL User Manual l iii INDEX Method invoking 3 10 min 2 13 3 12 4 15 Motif 1 5 N name 2 15 Named Collection 2 22 Objects 2 4 Query 2 14 Values 2 4 O O2 Architecture 1 2 OC 1 3 O Corba 1 3 O DBAccess 1 3 O2Engine 1 2 OsGraph 1 3 OsKit 1 3 OsLook 1 3 1 5 OeReport 1 3 Os5Store 1 2 OsTools 1 3 OWeb 1 3 Object Creation 2 12 Named 2 4 Objects 4 6 ODMG model 3 3 ODMG standard 32 4 1 Operation 4 12 Operator 2 13 3 12 2 20 2 20 2 20 Aggregative 3 12 avg 2 13 3 12 4 15 Combining 2 12 2 22 Composition 3 12 count 2 13 3 12 4 15 define 2 14 3 12 element 2 14 except 2 20 3 12 exists 2 15 3 12 flatten 2 21 forall in 3 12 group by 2 15 3 12 3 13 4 18 intersect 2 20 3 12 like 2 19 max 2 13 3 12 4 15 min 2 13 3 12 4 15 order by 2 19 4 20 Set 2 20 2 22 3 12 sum 2 13 3 12 4 15 union 2 20 3 12 Wild card 2 22 OQL 1 3 1 4 2 1 Operators 2 13 Rational 3 1 Result 2 10 order by 2 19 3 12 4 20 P partition 2 16 Path expressions 2 8 3 7 Polymorphism 3 11 Predicate 3 8 l iv ODMG OQL User Manual INDEX Q Query Basic 2 2 4 32 Named 2 14 Result 2 10 2 14 R Ref 3 3 Relationship 3 3 3 7 4 11 S select 3 13 Select from where 4 16 select from where 2 6 3 7 Set 2 6 2 7 Constructing 4 7 List conversion 2 21 Operators 2 20 3 12 set 2 11 2 12 3 9 4 22 4 24 Set expression 4 35 Smalltalk 1 3 Smalltalk
13. 20 ODMG OQL User Manual Language Definition Indexed Collection Example 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 e and e are expressions e is a list or an array e2 is an integer then ei e2 is an expression This extracts the e 1 th element of the indexed collection e4 Notice that the first element has the rank O Example list a b c d 1 This returns b Example element select x from Courses x where x name math and x number 101 requires 2 This returns the third prerequisite of Math 101 4 3 11 2 Extracting a Subcollection of an Indexed Collection If e4 2 and e are expressions e is a list or an array e and e3 are integers then e e2 e3 is an expression This extracts the subcollection of e starting at position e and ending at position e3 Example list a b c d 1 3 Thisreturns list b c d ODMG OQL User Manual 4 21 4 3 11 3 4 3 11 4 4 3 12 Example element select x from Courses x where x name math and x number 101 requires 0 2 This returns the list consisting of the first three prerequisites of Math 101 Geiting the First and Last Elements of an Indexed Collection If e is an expression if op is an operator from first last eisa list or a
14. a bag of the name of Globe clients Note Note 2 3 The query returns a bag and not alist To return alist you must define an order See Order by on page 19 You can also add the keyword distinct toa selection to eliminate any duplicates from the resulting set You can manipulate very complex structures A list can be made up of tuples which in turn can have a set attribute etc Consequently you have access to all the embedded components of an object For more details refer to Section 2 3 for constructing query results and Section 2 7 for combining operators Constructing results The structure of a query result is very often implicit For example when you extract the age field of an employee which is of type integer you obtain an integer When you filter a set bag or list you obtain a set bag or list depending on what you select ODMG OQL User Manual Constructing results However you can also construct a query result with an explicit structure using the struct set bag list and array constructors For example using the struct constructor select struct employee e name position e position salary e salary from e in Globe employees or simply select e name e position e salary from e in Globe employees This query gives the name position and salary of the employees at the International Globe newspaper ofshell You can use the sp
15. assuming that vectint is the name of a class whose type is Bag lt int gt 4 3 4 2 Constructing Structures If Pi P2 Panare property names and e eo eare expressions then struct py e1 po Go Pn en is an expression It defines the structure taking values e e2 e on properties py po Pn Note that this dynamically creates an instance of the type struct p1 ti Po to Pn ty if t is the type of ei Example struct name Peter age 25 This returns a structure with two attributes name and age taking respective values Peter and 25 See also abbreviated syntax in some contexts in Section 4 4 1 4 6 ODMG OQL User Manual Language Definition Construction Expressions 4 3 4 3 Constructing Sets lf ei eo areexpressions then set eq eo ej isan expression It defines the set containingtheelements e1 e2 en It creates a set instance Example set 1 2 3 This returns a set consisting of the three elements 1 2 and 3 4 3 4 4 Constructing Lists If ei e2 e are expressions then list ei eo ey or simply ei e2 eg are expressions They define the list having elements e1 e2 e They create a list instance If min max are two expressions of integer or character types such that min max then list min max or simply min max are expressions whose value is list min min 1 max 1 max
16. 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 This returns a bag of objects of type couple giving student names and the names of the full professors from which they take classes Example select from Students as x x takes as y y taught_by as z where z rank full professor select This returns a bag of structures giving for each student object the section object followed by the student and the full professor object teaching in this section bag lt struct x Student y Section z Professor gt Iterator Variables A variable x declared in the from part ranges over the collection e and thus has the type of the elements of this collection Such a variable can be used in any other part of the query to evaluate any other expressions see the Scope Rules in Section 4 3 15 Syntactical variations are accepted for declaring these variables exactly as with SQL The as keyword may be omitted Moreover the variable itself can be omitted and in this case the name of the collection itself serves as a variable name to range over it ODMG OQL User Manual 4 17 Example select couple student Students name professor z name from Students Students takes y y taught by z where z rank full professor O gt note In Os an additional syntax is allowed to declare a variable x fr
17. defined on objects for that purpose and thus does not breach the semantics of an Object Database which by definition is managed by the methods defined on the objects OQL provides declarative access to objects Thus OQL queries can be easily optimized by virtue of this declarative nature The formal semantics of OQL can easily be defined 4 2 ODMG OQL User Manual Language Definition Query Program 4 3 4 3 1 Language Definition OQL is an expression language A query expression is built from typed operands composed recursively by operators We will use the term expression to designate a valid query in this section Query Program A query program consists of a possibly empty set of query definition expressions followed by an expression which is evaluated as the query itself The set of query definition expressions is non recursive although a query may call an operation which issues a query recursively For example define jones as select distinct x from Students x where x name Jones select distinct student_id from jones This defines the set jones of students named Jones and evaluates the set of their student_ids O gt note With the Os query interpreter you use CTRL D on Unix or CTRL Z On Windows between two queries rather that 4 3 2 Named Query Definition If qis an identifier and e is a query expression then define q as eis a query definition expression which de
18. iai s 41 e 3 10 POR MOCO RT ET m 3 11 LOREM compos oen oOdpetopmenta hi e eH EVURPEEMCIREP E PEDIR 3 12 OQL REFERENCE 4 1 A1 iii looo o m c 4 2 A 4 2 4 3 Language Dein ODLas seen ivispen ard OA AUR D DU VD ER a EUN GR dE 4 3 LONE Rey INTERRETE NND 4 3 43r Named Query DSUBDOR nbi I ecd udis is ua eU 4 3 43o Elementary Expressions eee Ones 4 5 43a Atomie Literal Sna 4 5 Aaaa Named OGOS oe E 4 5 LAE ir ab i T Jes 4 5 LX oe Named QUE 4 5 224 0n 9n DD Expressions een ema ERI EH EPI RE EIE E 4 6 LET MS irai Er ori OPE 4 6 8 3412 Constructing SICUCEDUEBS eisini 4 6 8 3453 CONSITHCDDE SOlS aas ap be do M MEM e UP MM dab p MM bM 4 7 ODMG OQL User Manual iii G2 TABLE OF CONTENTS BDA A Constructing M m S 4 7 ID SN STS Bags Lupi PR EE seu DUM US t MENU 4 8 4 3 4 6 Constructing AXTAVS sinisisid 4 8 4 3 5 Atomie Types EXpressiONS ennonn 4 8 223 nary Expres Scen a M sU M 4 8 daad Binary ESPOSOS MINIM Tr 4 9 LAN E PROTONS e m 4 9 AO Object ESES IO PE 4 11 4 3 6 1 Comparison of Mutable Objects 4 11 4 3 6 2 Comparison of Immutable Objects 4 11 4 3 6 3 Extracting an Attribute or Traversing a Relationship from sigo s eet 4 11 4 3 6 4 Applying an Operation to an Object 4 12 4 3 6 5 Applying an Operation with Parameters to an Object 4
19. name Globe Company constant name the employees o2 set Employee ODMG OQL User Manual 2 3 2 GETTING STARTED Database entry points To query any database you need various entry points In Os these are the named objects and named values For example Globe is an entry point The simplest OQL query calls an entry point This returns jf The International Globe In an Op database named objects and values can either be values of any type or objects of any class Consequently OQL allows you to query values or objects of any type or class Note The query results shown below are all given in the Unix graphic form 2 4 ODMG OQL User Manual Basic queries Simple queries Simple queries Simple queries can involve different types of values e Atomic values With atomic values you can carry out arithmetic calculations e g This is a query which returns the integer 4 e Struct values You can also consider the value of the object Globe of class Company as a struct or tuple value with three attributes The only operation you can carry out on a struct is extracting a field e g This returns the name of the Globe Company ODMG OQL User Manual 2 5 2 GETTING STARTED List or array values A list is an ordered collection that allows duplicates and you can therefore ex
20. next sub object It is only valid if the Back button has been activated at least once New Window This button displays the current object in a new window Each window is an independent browser Quit This button closes the active window The query result is an object of the Person class which has a name an age and a spouse A spouse is also an object of the Person class and thus appears in as a hypertext link Left clicking displays the spouse object Note The rest of this manual will only show graphical displays from the Unix platform ODMG OQL User Manual Interactive and embedded query language 1 2 Interactive and embedded query language It is because OQL is so easy to use interactively that all kinds of users including non technical users can browse the database quickly and efficiently to get the information they want OQL can also be used as a function called from C C Smalltalk Java OoC and Op Engine API Interactive OQL The OQL interpreter can be triggered by the query command of O2dba O2dsa or O shells The command interpreter prompts you with the following message type your command and end with D To run OQL type query D You must type D Control D on a separate line You now see Query Interpreter type your query and end with D Type your query ending it with D this is a query D The answer is automatically displayed and the system returns to the OQL prompt
21. query may test explicitly if an object is different from nil before accessing a property Example Doe name This returns Doe ODMG OQL User Manual 4 11 Example This returns true if Doe has a spouse whose name is Carol or else false O gt note 4 3 6 4 4 3 6 5 According to a recent evolution of the ODMG standard OQL does not now raise an exception when it traverses a path which contains a ni1 Instead of this a predicate involving such a path is always false This means that OQL now skips such elements and thus the explicit test to nil is not yet mandatory Applying an Operation to an Object If e is an expression if is an operation name then e gt f and e f are expressions These are alternate syntax to apply on operation on an object The value of the expression is the one returned by the operation or else the object nil if the operation returns nothing Example jones number of students This applies the operation number of students to jones Applying an Operation with Parameters to an Object If e is an expression if e e e are expressions if fis an operation name then e gt f e e ej and e f e e e are expressions that apply operation with parameterse e e to object e The value of the expression is the one returned by the operation or else the object nil if the operation returns nothing In both cases if ehappe
22. set 7 This returns the set containing 1 2 3 4 5 6 7 flatten list list 1 2 list 1 2 3 This returns 1ist 1 2 1 2 3 flatten set list 1 2 1ist 1 2 3 This returns the set containing 1 2 3 ODMG OQL User Manual 4 25 4 3 13 5 4 3 14 4 3 15 Typing an Expression If e is an expression if cis atypename then c eis an expression This asserts that e is an object of class type c If it turns out that it is not true an exception is raised at runtime This is useful to access a property of an object which is statically known to be of a superclass of the specified class Example select Employee s salary from Students s where s in select sec assistant from Sections sec This returns the set of salaries of all students who are teaching assistants assumingthat Students and Sections arethe extents of the classes Student and Section Function Call If f isa function name ife1 e2 e are expressions then f and f el e2 en are expressions whose valueis the value returned by the function or the object nil when the function does not return any value The first form allows you to call a function without a parameter while the second one calls a function with the parameters e eo e OQL does not define in which language the body of such a function is written This feature allows you to smoothly extend the functionality of OQL without chang
23. the struct syntax In the group by part Op accepts the 3 alternatives but does not accept the struct syntax ODMG OQL User Manual 4 29 4 4 2 Aggregate Operators These operators were introduced in Section 4 3 7 4 SQL adopts a notation which is not functionnal for them So OQL accepts this syntax too If we define aggregate as one of min max count sum and avg select count from is equivalent to count select from select aggregate query from is equivalent to aggregate select query from select aggregate distinct query from is equivalent to aggregate distinct select query from O note O does not support Aggregate Operator abbreviations 4 4 3 Composite Predicates If e4 and e are expressions e is a collection e has the type of its elements if relation is a relational operator lt lt gt gt then e relationsomee and e relation any e and e relation all es are expressions whose value is a boolean The two first predicates are equivalent to exists x in e5 e relation x The last predicate is equivalent to for all x in eg e relation x Example 10 some 8 15 7 22 is true 4 30 ODMG OQL User Manual Syntactical Abbreviations String Literal O gt note In O gt Composite Predicate abbreviations are not supported 4 4 4 String Literal OQL accepts simple quotes as well to delimit a str
24. then S like pattern is an expression of type boolean whose value is true if s matches the pattern else false Example a nice string like nice str ng is true Os note wee In Os the only supported wildcard is ODMG OQL User Manual Language Definition Object Expressions 4 3 6 Object Expressions 4 3 6 1 Comparison of Mutable Objects If e and e are expressions which denote mutable objects objects with identity of the same type then e e and e4 e 1 2 1 2 are expressions which return a boolean The second expression is equivalent to not eq ez e ejis true if they designate the same object Example Doe element select s from Students s where s name Doe is true 4 3 6 2 Comparison of Immutable Objects If e4 and eg are expressions which denote immutable objects literals of the same type then e17 eo and ey e2 are expressions which return a boolean the second expression is equivalent to not eq ej e1 e is true if the value e is equal to the value eg 4 3 6 3 Extracting an Attribute or Traversing a Relationship from an Object If eis an expression if pis a property name then e gt p and e p are expressions These are alternate syntax to extract the property p of an object e If e happens to designate a deleted or a non existing object i e nil an attempt to access the attribute or to traverse the relationship raises an exception However a
25. 0 This returns true if all the objects in the Students set have a positive value for their student id attribute Otherwise it returns false Existential Quantification If x is a variable name if e4 and e are expressions e4 denotes a collection and e a predicate then exists x in e eo is an expression It returns true if there is at least one element of collection e that satisfies e and false otherwise Example exists x in Doe takes x taught by name Turing ODMG OQL User Manual Language Definition Collections Expressions This returns true if at least one course Doe takes is taught by someone named Turing If eis acollection expression then exists e and unique e are expressions which return a boolean value The first one returns true if there exists at least one element in the collection while the second one returns true if there exists only one element in the collection Notice that these operators allow the acceptance of the SQL syntax for nested queries such as select from col where exists select from col where predicate The nested query returns a bag to which the operator exists is applied This is of course the task of an optimizer to recognize that it is useless to compute effectively the intermediate bag result O gt note In Os these two last operations are not supported Only the form exists x in el e2 is valid 4 3 7 3 Membership Testing If e4 and
26. 35 F Hasher BAL24 440BF Assembled unit SADH8 Fixing set Figure 1 2 Typical OQL query result in graphical form as generated in Unix In addition to the usual Motif buttons a graphical query result has an Eraser button Clicking on the Eraser button removes the graphical result This query result consists of a number of objects Each object has its own pop up menu which is displayed by clicking the Object icon using the right mouse button This pop up menu can be used to access the public methods of each object ODMG OQL User Manual 1 5 INTRODUCTION Windows NT In Windows NT the query result is displayed in a window in textual form containing hypertext links Each link represents a sub object The label for a specific link may be obtained by applying the title method to the sub object represented by the link Clicking on a hypertext link with the right mouse button replaces the contents of the window with a representation of the sub object associated with the link Figure 1 3 shows a typical query result in graphical form as generated in Windows NT S oz OGL Query nom Joseph age 30 epoux Person aia ao en oor Figure 1 3 Typical OQL query result in graphical form as generated in Windows NT The browser shown in Figure 1 3 has the following buttons Back this button displays the previous object Forward this button displays the
27. B DEL isse ati quib bee Uma 4 24 4 3 133 Removing Duplioales nsa iisc cix tn bb pk lad cu kbbn Edita pe 4 25 4 3 13 4 Flattening Collection of Collections 4 25 45 13 Typing un EXpressipllas ieiunio abb dU sc ei 4 26 SaL Foncion Call e 4 26 6 2 15 Se oo ce Y 4 26 Syntactical Abbreviations sees 4 28 AA EDDIE COOREEUEBOR e uos diss deti cna NES ela SUR cU iU didi ed 4 29 SAI PP ate CDI MG e eatis nd absum nos bita dU qM 4 30 A43 Composite Predicates ubi id eb pet MP pH iDM anaita 4 30 A04 SEDE Lier e bp patrie buddies eb D NIME ME 4 31 OL BNP m 4 32 LAN E 0 m 4 32 4 5 1 1 Axiom see Sections 4 3 1 4 3 2 sesesess 4 32 4 5 1 2 Basic see Section 43 3 see mee t i Ra EUR 4 32 4 5 1 3 Simple Expression see Section 4 3 5 4 33 4 5 1 4 Comparison see Section 4 3 5 4 33 4 5 1 5 Boolean Expression see Section 4 3 5 4 33 4 5 1 6 Constructor see Section 4 3 4 ssuuss 4 33 4 5 1 7 Accessor see Sections 4 3 6 4 3 11 4 3 14 4 3 15 4 34 4 5 1 8 Collection Expression see Sections 4 3 7 4 4 3 4 34 4 5 1 9 Select Expression see Sections 4 3 8 4 3 9 4 3 10 4 34 ODMG OQL User Manual V G2 TABLE OF CONTENTS 4 5 1 10 Set
28. Client collection o2 list Client list Client name John Client name Jack 2 4 Operators Count This section outlines the basic OQL operators you can use to query the database You can query the database using the count clause For example to find out how many employees there are at the International Globe newspaper count Globe employees This query returns an integer Other aggregate operators are min max sum and avg ODMG OQL User Manual 2 13 2 GETTING STARTED Define You can name the result of a query using the define clause For example define MyEmployees as select e from e in Globe employees where e name like Sp This names the result of the query and not the query itself The name MyEmployees can then be used in other queries Named queries greatly improve the legibility of complex queries Note You can only reuse these named queries in the same query session i e up to a commit or abort point Element When you have a set or a bag that contains a single element you extract the element directly using the element operator For example element select e from e in Globe employees where e name Tintin This query gives the result Tintin 2 14 ODMG OQL User Manual Operators Exists Exists You can add a new persistent name to cover all the different compa
29. Example list 1 2 2 3 This returns a list of four elements Example list 3 5 This returns the list 3 4 5 Ov note In Os the keyword list is mandatory ODMG OQL User Manual 4 7 4 3 4 5 Constructing Bags lf e4 eo are expressions then bag e eo ej isan expression It defines the bag having elements e e2 ey It creates a bag instance Example bag 1 1 2 3 3 This returns a bag of five elements 4 3 4 6 Constructing Arrays lf ei e2 eare expressions then array ei e2 e isan expression It defines an array having elements e e2 eg It creates an array instance Example array 3 4 2 1 1 This returns an array of five elements 4 3 5 Atomic Types Expressions 4 3 5 1 Unary Expressions If eis an expression and op is a unary operation valid for the type of e then op eis an expression It defines theresult of applying op to e Arithmetic unary operators are abs Boolean unary operator is not Example not true This returns false 4 8 ODMG OQL User Manual Language Definition Atomic Types Expressions 4 3 5 2 Binary Expressions If e and e are expressions and op is a binary operation then e lt op gt e is an expression It defines the result of applying op to e4 and eo Arithmetic integer binary operators are mod modulo Floating point binary o
30. Expression see Section 4 3 12 4 35 4 5 1 11 Conversion see Section 4 3 13 4 35 Boe Operator POOR ariora lop EEN 4 35 INDEX l i vi ODMG OQL User Manual 1 INTRODUCTION Congratulations You are now a user of the object oriented query language OQL Op is a revolutionary system that is particularly well adapted for developing large scale client server applications in both fields of business and technical software development This chapter introduces the O system and the OQL query language It is divided up into the following sections System Overview Interactive and embedded query language e Manual overview ODMG OQL User Manual 1 INTRODUCTION 1 1 System Overview The system architecture of Oz is illustrated in Figure 1 1 External Interfaces Development Tools Ov Dev Tools C C Standard Dev Tools OQL OC Java O Corba O DBA OsgEngine O5Store Cane Figure 1 1 Oo System Architecture The Os system divided into three components The Database Engine provides all the features of a Database system and an object oriented system This engine is accessed with Development Tools such as various programming languages O gt development tools and any standard development tool Numerous External Interfaces are provided All encompassing Oo is a versatile po
31. From a name an application can directly retrieve the named object and then operate on it or navigate to other objects following the relationship links Let us now present the model through a complete example We usehere C syntax for our object definition language following the ODMG C ODL binding i e the way of defining an ODMG schema using the standard C language ODMG OQL User Manual 3 3 OQL RATIONALE class Person d_String name d_Date birthdate d Set lt d_Ref lt Person gt gt parents inverse children d_List lt d_Ref lt Person gt gt children inverse parents d_Ref lt Apartment gt lives_in inverse is_used_by Methods Person Constructor a new Person is born int age Returns an atomic type void marriage d_Ref lt Person gt spouse This person gets a spouse void birth d_Ref lt Person gt child This person gets a child d Set d_Ref lt Person gt gt ancestors Set of ancestors of this Person virtual d_Set lt d_String gt activities A redefinable method class Employee Person A subclass of Person float salary Method virtual d_Set lt d_String gt activities This method is redefined 3 4 ODMG OQL User Manual The ODMG model class Student Person A subclass of Person d_String grade Method virtual d_Set lt d_String gt activities The method is redefined ODMG OQL User Manua
32. RL OIN uus WNETE La oa sansa nites 2 6 2 M XY 2 7 Poo eT P 2 8 FUIL OCDE od onis caduti MM Cebu acad PRET M 2 8 Wc s po TRU Wn aoa eels 2 9 I D Ein P cr e 2 9 2 3 Consirucling resus ccceiiccssaasasasesdcsincnssanaiaddnnancendorinaianianceniane 2 10 Creatine nis dio TPTCT 2 12 it T 2 13 oo e ee oP eB rec SNF oe 2 13 DEG 2 14 Eleme TENER D 2 14 eet see a eee 2 15 LEONE D ou pb Quedadas erat nent eres 2 15 LARS 2 19 doc m 2 19 EE qe ini Me 2 20 26 CONGO ODD e HH 2 21 E Rope 2 21 inju 2 21 ODMG OQL User Manual TABLE OF CONTENTS lur ess eee eee 2 21 2 7 Combining operators sicisssscdaissicanisicietindanadaneidseatindnaddadannaune 2 22 ZB NE 2iiidedpbiddiciidibaeidlckuikibib mia ad om Ma Kol abidl 2 22 Display ide fem TNNT HP EDEN ETINDENE 2 23 2 9 Chapter SUMMARY aeoiieuasas dla vekk dit idhiblddidda pui i nasaia 2 24 OQL RATIONALE 3 1 21 The ODMG stahdald Mee 3 2 ag Ee cg cncnidnnrninan 3 3 26 UL BY GX SIND IS iioc eR eRVIE IHE RD EUNKFACI A FPE OL DFU DG ERH ERN 3 7 ly ngon jo ce casein 3 7 Data manipulation M 3 9 bu
33. We have now to traverse two collections Persons and Person children Like in SQL the select from operator allows us to query more than one collection These collections then appear in the from part In OQL a collection in the from part can be derived from a previous one by following a path which starts from it and the answer is select c lives in building address from p in Persons c in p children This query inspects all children of all persons Its result is of the type Bag lt Address gt Predicate Of course the where clause can be used to define any predicate which then serves to select the data matching the predicate For instance to restrict the previous result to the people living on Main Street and having at least 2 children who do not live in the same apartment as their parents the query 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 gt 2 and c lives_in p lives in Join In the from clause collections which are not directly related can also be declared As in SQL this allows us to compute joins between these collections For instance to find the people living in a street and having the same name as this street we do the following the Building extent is not defined in the schema so we have to compute it from the Apartments extent To compute this intermediate result we need a sel
34. ark of the NeXT Computer Inc Purify Quantify are registered trademarks of Pure Software Inc Windows is a registered trademark of Microsoft Corporation All other company or product names quoted are trademarks or registered trademarks of their respective trademark holders Who should read this manual OQL is an object oriented SQL like query language the ODMG standard This manual describes how to use OQL as an embedded function in a programming language e g O2C C C or Smalltalk or interactively as a query language It assumes previous knowledge of the O system Other documents available are outlined click below See O Documentation set TABLE OF CONTENTS This manual is divided into the following chapters 1 Introduction 2 Getting Started Qus 4 OQL Reference OQL Rationale ODMG OQL User Manual G2 TABLE OF CONTENTS INTRODUCTION 1 1 T Sysiom COVE Wo soiien eain nina aeisi 1 2 DERE owetintbi but oui Gare nearest eee 1 4 Browser Interlace MMCT 1 5 1 2 Interactive and embedded query language 1 7 Interactive OQU T 1 7 Embedded QU PROP T s 1 8 13 Manual OVOPVIBWL ii deae E P kdadua as sd e iaasa 1 8 GETTING STARTED 2 1 Hg ci 1 5 2 2 Database enit Y POMS e pedes hidalgo PE Hn PLU MIN ines 2 4 SUBDIE HUE iva noie M us eA MEE M MM RE NIRE 2 5 2 2 B
35. at from cl in Globe clients ord in cl order where cl name Haddock You obtain the set of what the client s called Haddock bought Booklet ODMG OQL User Manual Select from where Testing on nil Testing on nil After your application has updated the database you may find that some objects are now equal to nil You can test for this using OQL For example you can test that a client exists and if so which client has three orders select c name from c in Globe clients where c nil and count c order To simplify programming OQL skips nil objects when they are encountered If a path expression contains a nil object a predicate is always considered false This means that the previous expression can be rewritten as follows select c name from c in Globe clients where count c order List or array A list or an array is an ordered collection that can contain duplicate elements Sinceit is ordered you may extract any of its elements if you know their position For example Globe clients 2 This extracts the third element of the list the first element is at position 0 As with sets you can filter a list ODMG OQL User Manual 2 9 GETTING STARTED For example what are the names of the clients who buy the International Globe newspaper select e name from e in Globe clients The result of this query is
36. binding 3 2 struct 2 11 2 12 2 12 3 9 Struct value 2 5 structure 4 6 Subcollection 4 21 4 21 sum 2 13 3 12 4 15 System Architecture 1 2 Testing on nil 2 9 Typing 4 26 U union 2 20 3 12 4 23 Universal quantification 2 22 3 12 4 14 Unix 1 5 V Value Array 2 6 Atomic 2 5 List 2 6 Named 2 4 Struct 2 5 W where 3 8 Windows NT 1 6 ODMG OQL User Manual l v INDEX vi ODMG OQL User Manual
37. e are expressions es is a collection e has the type of its elements then ei in eo is an expression It returns true if element e belongs to collection e3 Example Doe in Does This returns true 4 3 7 4 Aggregate Operators If eis an expression which denotes a collection if op is an operator from min max count sum avg then op e is an expression Example max select salary from Professors This returns the maximum salary of the Professors ODMG OQL User Manual 4 15 4 3 8 Select From Where 4 3 8 1 If ei eg e are expressions which denote collections and x xp XQ are variable names if e is an expression of type boolean and if projection is an expression or the character then select projection from e as x e2 as x e as Xx where e and select distinct projection from e as x1 e9 as xj e as x where e are expressions The result of the query is a set for a select distinct or a bag fora select If you assume ey e2 e areall set and bag expressions then the result is obtained as follows take the cartesian product of the sets ei 892 eg filter that product by expression e i e eliminate from the result all objects that do not satisfy boolean expression e apply the projection to each one of the elements of this filtered set and get the result When the result is a set distinct case duplicates ar
38. e automatically eliminated The situation where one or more of the collections e e2 e S an indexed collection is a little more complex The select operator first converts all these collections into sets and applies the previous operation Theresult is a set distinct case or else a bag So in this case we simply transform each of the e s into a set and apply the previous definition Projection Before the projection the result of the iteration over the from variables is of type bag struct x type of e elements Xp type of e elements The projection is constructed by an expression which can then refer implicitly to the current element of this bag using the variables x If for e neither explicit nor implicit variableis declared then x is given an internal system name which is not accessible by the query anyway By convention if the projection is simply then the result of the selection is the same as the result of the iteration If the projection is distinct the result of the select is this bag converted into a set 1 The cartesian product between a set and a bag is defined by first converting the set into a bag and then getting the resulting bag which is the cartesian product of the two bags ODMG OQL User Manual Language Definition Select From Where 4 3 8 2 In all other cases the projection is explicitly computed by the given expression Example select
39. e enables you to extract those elements meeting a specific condition from a list or set Constructing results The structure of a query result is very often implicit However you can also construct a query result with an explicit structure using the struct set and list constructors Operators OQL operators include define element order by count exists group by and like They can be combined for complex queries e Indexes When OQL extracts one or more elements from a set or list it scans the whole collection to find the desired elements You can improve performance if you tell the system exactly where to look This is done by establishing an index on a collection An index maps a key to one or more elements of a named collection 2 24 ODMG OQL User Manual 3 OQL RATIONALE Most commercial object database systems now have a common data model based on the OMG object model This data model is defined in the ODMG 93 report Based on this ODMG model the query language OQL was defined and adopted by the ODMG group This chapter is divided up as follows The ODMG standard e The ODMG model e OQL by example ODMG OQL User Manual 3 1 OQL RATIONALE 3 1 The ODMG standard The ODMG standard covers the following points 1 an object model 2 an object definition language for this model with its own syntax ODL or its expression through C and Smalltalk syntax
40. e relationships For instance given a Person p to know the name of the street where this person lives we use the following OQL query p lives_in building adddress street This query starts from a Person traverses an Apartment arrives in a Building and goes inside the complex attribute of type Address to get the street name This example treated 1 1 relationship let us now look at n p relationships Assume we want the names of the children of the person p We cannot write p children name because children is a List of references so the interpretation of the result of this query would be undefined Intuitively the result should bea collection of names but we need an unambiguous notation to traverse such a multiple relationship and we use the select from where clause to handle collections just as in SQL select c name from c in p children The result of this query is a value of type Bag lt String gt If we want to get a Set we simply drop duplicates like in SQL by using the distinct keyword select distinct c name from c in p children Now we have a means to navigate from any object to any other object following any relationship and entering any complex subvalues of an object ODMG OQL User Manual 3 7 OQL RATIONALE For instance we want the set of addresses of the children of each Person of the database We know the collection named Persons contains all the persons of the database
41. ecial operator to select all attributes of the elements of a collection ODMG OQL User Manual 2 11 GETTING STARTED For example select from Globe employees Note that in this example you do not need to define a variable with from You can also build up embedded structures simply by combining struct operators For example to get the identities and salaries of all those employees working as reporters and older than 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 gt 22 This query gives a bag with one element 260 000000 Creating an object You create values using struct list array bag and set In OQL you can also create objects using the class name and by initializing the attributes of your choice Any un initialized attributes are set to the default value For example to create an object of the class Client Client name Trent This creates atemporary object with the name attribute initialized to Trent ODMG OQL User Manual Operators Count You can then make the object persistent in the usual way refer to the O5C C Smalltalk and Java manuals The result of this query is the new object An object collection can be created in the same way For example use the following query to create an 02 list
42. ect from operator again So the join is done as follows 3 8 ODMG OQL User Manual 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 This query highlights the need for an optimizer In this case the inner select subquery must be computed once and not for each person Data manipulation A major difference between OQL and SQL is that an object query language must manipulate complex values OQL can therefore create any complex value as a final result or inside the query as intermediate computation To build a complex value OQL uses the constructors struct set bag list and array For example to obtain the addresses of the children of each person along with the address of this person we use the following query 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 ODMG OQL User Manual 3 9 3 OQL RATIONALE This gives for each person the name the address and the name and address of each child The type of the result is a bag of the following struct struct String me Address my_address Bag lt struct String name Address address gt my_children OQL can also create complex objects For this pu
43. ents is defined as follows If the from clause declares the variables v4 on collection co1 v5 on colz v4 Oh col the grouped elements form a structure with one attribute v for each collection having the type of the elements of the corresponding collection partition bag struct v type of col elements va type of col elements If a collection co1 has no variable declared the corresponding attribute has an internal system name This partitioned set may then be filtered by the predicate of a having clause Finally theresult is computed by evaluating the select clause for this partitioned and filtered set The having clause can thus apply aggregate functions on partition likewise the select clause can refer to partition to compute the final result Both clauses can refer also to the partition attributes Example select from Employees e group by low e salary 1000 medium e salary gt 1000 and salary 10000 high e salary 10000 This gives a set of three elements each of which has a property called partition which contains the bag of employees that enter in this category So the type of the result is set struct low boolean medium boolean high boolean partition bag lt struct e Employee gt gt The second form enhances the first one with a having clause which enables you to filter the result using aggregative functions which operate on each partition ODMG OQL Use
44. ery query list query query query query query list query query array query query 1 The operator is also used for list and array concatenation ODMG OQL User Manual 4 33 4 5 1 7 Accessor see Sections 4 3 6 4 3 11 4 3 14 4 3 15 4 5 1 8 4 5 1 9 query query query query dot query query query query query query Collection query query query query query query quantifier quantifier quantifier query query query query query query query dot attribute_name query dot relationship_name query dot operation_name query dot operation_name query query gt query query query query query query first query last query function name query query Expression see Sections 4 3 7 4 4 3 forall identifierin query query exists identifier in query query exists query unique query query in query query comparison operator quantifier query some any all count query count sum query min query max query avg query 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 criter
45. fines the query with name q Example define Does as select x from Student x where x name Doe This statement defines Does as a query returning a bag containing all the students whose name is Doe define Doe as element select x from Student x where x name Doe This statement defines Doe as a query which returns the student whose nameis Doe if thereis only one otherwise an exception is raised ODMG OQL User Manual 4 3 O gt note define operation is available only with the interactive query interpreter It has no meaning for OQL embedded in programming languages C Smalltalk O2C because standard programming language variables can be used for that purpose A defined name is valid up to the next commit or abort You can get the list of current defined queries by typing the query display queries 4 4 ODMG OQL User Manual Language Definition Elementary Expressions 4 3 3 Elementary Expressions 4 3 3 1 Atomic Literals If lis an atomic literal then is an expression whose value is the literal itself Literals have the usual syntax Object Literal nil Boolean Literal false true Integer Literal sequence of digits e g 27 Float Literal mantissa exponent The exponent is optional e g 3 14 or 314 16e 2 Character Literal character between simple quotes e g z String Literal character string between double quote e g a string
46. ies details how to construct results and describes the use of operators and indexes To fully understand this chapter you must know the ODMG data model Chapter 3 OQL Rationale This chapter introduces the ODMG standard and describes the ODMG object model It also gives an example based presentation of OQL Chapter 4 OQL Reference This chapter contains the ODMG reference manual for OQL 1 2 It is the same as the ODMG standard with added notes and explanations on how to use OQL with Os For each feature of the language you get the syntax in informal semantics and an example Finally the formal syntax is given 1 8 ODMG OQL User Manual D GETTING STARTED AN OBJECT ORIENTED DATABASE QUERY LANGUAGE So that you can obtain the exact information you want from your database O has an object oriented database query language OQL OQL is a powerful and easy to use SQL like query language with special features for dealing with complex objects values and methods This chapter introduces the OQL language and is divided up into the following sections Basic queries Select from where Constructing results Operators e Set operators e Conversions Combining operators Indexes Chapter Summary To understand this chapter you need to know the ODMG data model As an introduction to the data model you can refer to chapter 3 of this manual or the OoC Beginner s Guide
47. ing see Section 4 3 3 1 as SQL does This introduces an ambiguity for a string with one character which then has the same syntax as a character literal This ambiguity is solved by context O gt note In Oo a string must be delimited by double quotes ODMG OQL User Manual 4 31 4 5 4 5 1 4 5 1 1 4 5 1 2 OQL BNF The OQL grammar is given using a BNF like notation e symbol means a sequence of 0 or more symbol s e fsymbol means an optional symbol Do not confuse with the separators keyword is a terminal of the grammar Keywords are not case sensitive e xxx name has the syntax of an identifier e xxx literal is self explanatory e g a string is a string literal bind argument stands for a parameter when embedded in a programming language e g 3i The non terminal query stands for a valid query expression The grammar is presented as recursive rules producing valid queries This explains why most of the time this non terminal appears on the left side of Of course all operators expect their query operands to be of the right type Type constraints were discussed in the previous sections These rules must be completed by the priority of OQL operators which is given after the grammar Some syntactical ambiguities are solved semantically from the types of the operands Grammar Axiom see Sections 4 3 1 4 3 2 query program define_query query define que
48. ing the language Scope Rules The from part of a select from where query introduces explicit or implicit variables to range over the filtered collections An example of an explicit variable is select from Persons p while an implicit declaration would be select from Persons The scope of these variables reaches all parts of the select from where expression including nested sub expressions The group by part of a select from where group by query introduces the name partition along with possible explicit attribute names which characterize the partition These names are visible in the corresponding having and select parts including nested sub expressions within these parts 4 26 ODMG OQL User Manual Language Definition Scope Rules Inside a scope you use these variable names to construct path expressions and reach properties attributes and operations when these variables denote complex objects For instance in the scope of the first from clause above you access the age of a person by p age When the variable is implicit as in the second from clause you use the name of the collection directly Persons age However when there is no ambiguity you can use the property name directly as a shortcut without using the variable name to open the scope this is made implicitly writing simply age Thereis no ambiguity when a property name is defined for one and only one object denoted by a visible variable
49. ion projection projection projection projection projection attributes projection projection attributes query identifier query query aS identifier variable declaration query as identifier 4 34 ODMG OQL User Manual OQL BNF Operator Priorities 4 5 1 10 4 5 1 11 4 5 2 partition_attributes projection projection sort criterion query ordering ordering asc ordering desc Set Expression see Section 4 3 12 query query intersect query query query union query query query except query Conversion see Section 4 3 13 query listtoset query query element query query distinct e query flatten query query class_name query Operator Priorities The following operators are sorted by decreasing priority Operators on the same line have the same priority and group left to right 0 D gt not unary unary in mod intersect union except lt gt lt gt lt some lt any lt all etc for all comparison operators l like and exists for all or identifier this is the cast operator order having group by where from select ODMG OQL User Manual 4 35 4 36 ODMG OQL User Manual INDEX ODMG OQL User Manual INDEX Symbols 2 20 2 20 A Accessor 4 34 Addition of sets 2 20 Aggrega
50. itizen Kane intin Kent The select clause defines the query result as the employees and the from clause gives the set on which to run the query The variablee represents each of its elements in turn The where clause filters the employees so that those earning more than 200 are extracted This query therefore builds a collection of employees This collection is in fact a bag as duplicates are accepted You can also add the keyword distinct to eliminate any duplicates from the resulting bag and then produce a true set Moreover you can access from e any attributes e g salary and get a set of real numbers For example select distinct e salary from e in Globe employees where e position Reporter This gives a set of the salaries of the Reporters ODMG OQL User Manual 2 7 GETTING STARTED Join B40 000000 BBO 060000 You can also use a query to select from more than one collection select e from e in Globe employees c in Globe clients where e name c name This query returns the set of employees who have the same name as a client If there is a client called Kent and an employee called Kent you see the following window Path expressions Objects are related to other objects and in order to get to the data it needs a query can follow various paths that start from any O object or collection For example select distinct ord wh
51. l 3 5 OQL RATIONALE class Address int number d String street class Building Address address A complex value Address embedded in this object d_List lt lt d_Ref lt Apartment gt gt apartments inverse building d_Ref lt Apartment gt less expensive class Apartment int number d Ref Building building d Ref Person is used by inverse lives in d Ref Person gt Persons All persons and employees d Ref Apartment gt Apartments The Apartement class extent d_Ref lt Apartment gt gt Vacancy The set of vacant appartements d_List lt d_Ref lt Apartment gt gt Directory The list of appartements ordered by their number of rooms 3 6 ODMG OQL User Manual OQL by example Path expressions 3 3 OQL by example Let us now turn to an example based presentation of OQL We use the database described in the previous section and instead of trying to be exhaustive we give an overview of the most relevant features Path expressions As explained above one can enter a database through a named object but more generally as soon as one gets an object which comes for instance from a C expression one needs a way to navigate from it and reach the right data one needs To do this in OQL we use the or indifferently gt notation which enables us to go inside complex objects as well as to follow simpl
52. n array then op e is an expression This extracts the first and last element of a collection Example first element select x from Courses x where x name math and x number 101 requires This returns the first prerequisite of Math 101 Concatenating Two Indexed Collections If e and e are expressions if e4 and e are both lists or both arrays then e e is an expression This computes the concatenation of e and e Example list 1 2 list 2 3 This query generates list 1 2 2 3 Binary Set Expressions 4 22 ODMG OQL User Manual Language Definition Binary Set Expressions 4 3 12 1 4 3 12 2 Union Intersection Difference If e and e are expressions if op is an operator from union except intersect if e and e are sets or bags then e4 op e is an expression This computes set theoretic operations union difference and intersection on e4 and es as defined in Chapter 2 When the collection kinds of the operands are different bag and set the set is converted into a bag beforehand and the result is a bag Examples Student except Ta This returns the set of students who are not Teaching Assistants bag 2 2 3 3 3 union bag 2 3 3 3 This bag expression returns bag 2 2 3 3 3 2 3 3 3 bag 2 2 3 3 intersect bag 2 3 3 3 The intersection of 2 bags yields a bag that contains the minimum for each of the multiply values So the result is bag
53. nies that exist name TheCompanies list Company You can now carry out more complex queries such as selecting which company has at least one employee under the age of 23 select c name from c in TheCompanies where exists e in c employees e age lt 23 The answer is a bag of names Group by This operator groups together objects of a collection with the same value for particular attributes For example select from e in Globe employees group by e salary This groups the employees by salary giving a bag of two attribute tuples ODMG OQL User Manual 2 15 GETTING STARTED o2sfvall The first attribute is the salary and is called salary as specified The second is the set of objects employees with the same salary and is called partition Thus the type of the result of this query is bag struct salary real partition bag struct e Employee ODMG OQL User Manual Operators Group by You can work on a partition value by computing statistics on each partition The following query returns a bag of two attribute tuples with the salary and the number of employees earning each of these salaries select salary number count partition from e in Globe employees group by e salary You get the following type of window Finally you can filter the result of grouping by applying predicates on aggregative operations You can select groups with condi
54. nion intersect intersection and except difference You can also write these operators as union intersection and difference You can define another query YourEmployees define YourEmployees as select e from e in Globe employees where e name Tintin Now you can combine the queries by adding together two sets MyEmployees YourEmployees The simple addition union of the two sets of employees gives you a set containing the answer The pick operator is defined on a set or a bag It returns an element of the collection chosen arbitrarily For example pick MyEmployees 2 20 ODMG OQL User Manual Conversions 2 6 Conversions List to set To convert a list or array to a set you use the listtoset operator Example listtoset Globe clients intersect listtoset TheCompanies 2 clients Set to list Flatten To convert a set or bag to alist you must order it For example select e from e in the employees order by e salary returns a list sorted by salary You can also use to build a list This avoids a real sort algorithm and should be used when the final order of the list is unimportant select e from e in the employees order by returns a list of all employees in random order To convert a collection of collections into a flattened collection you use the flatten operator For example flatten select distinct c client
55. ns to designate a deleted or anon existing object i e nil an attempt to apply an operation to it raises an exception However a query may test explicitly if an object is different from nil before applying an operation ODMG OQL User Manual Language Definition Object Expressions Example Doe gt apply course Maths Turing gt number This query calls the operation apply course on class Student for the object Doe It passes two parameters a string and an object of class Professor The operation returns an object of type Course and the query returns the number of this course ODMG OQL User Manual 4 13 4 3 6 6 4 3 7 4 3 7 1 4 3 7 2 Dereferencing an Object If eis an expression which denotes an object with identity a mutable object then eis an expression which delivers the value of the object a literal Example Given two variables of type Person p1 and p2 the predicate pl p2 is true if both variables refer to the same object while pl p2 is true if the objects have the same values even if they are not the same objects Collections Expressions Universal Quantification If xis a variable name e and e are expressions e denotes a collection and e a predicate then for all x in e e is an expression It returns true if all the elements of collection e satisfy e and false otherwise Example for all x in Students x student id gt
56. om x in collection This syntax will also be included in the next release of the ODMG standard 4 3 8 3 Predicate In a select from where query the whereclause can be omitted with the meaning of a true predicate 4 3 9 Group by Operator If select query is a select from where query partition attributes is a structure expression and predicate a boolean expression then select query group by partition attributes is an expression and select query group by partition attributes having predicate is an expression The cartesian product visited by the select operator is split into partitions For each element of the cartesian product the partition attributes are evaluated All elements which match the same values according to the given partition attributes belong to the same partition Thus the partitioned set after the grouping operation is a set of structures each structure has the valued properties for this partition the valued partition attributes completed by a property which is conventionally called partition and which is the bag of all objects matching this particular valued partition ODMG OQL User Manual Language Definition Group by Operator If the partition attributes are att e att e2 atta e then the result of the grouping is of type set struct att type of ej att5 type of e2 att type of e partition bag type of grouped elements The type of grouped elem
57. perators are 7 Relational binary operators are l lt lt gt gt These operators are defined on all atomic types Boolean binary operators are and or Example count Students count TA This returns the difference between the number of students and the number of TAs 4 3 5 3 String Expressions If s and s are expressions of type string then s s2 and s sz are equivalent expressions of type string whose value is the concatenation of the two strings Ov note In Os the operator is not accepted To concatenate 2 strings use If cis an expression of type character and s an expression of type string then c in s is an expression of type boolean whose value is true if the character belongs to the string else false ODMG OQL User Manual 4 9 If s is an expression of type string and iis an expression of type integer then s i is an expression of type character whose value is the i 1th character of the string If s is an expression of type string and 1ow and up are expressions of type integer then s low up is an expression of type string whose value is the substring of s from the low 1 th character up to the up 1 th character If s is an expression of type string and pattern a string literal which may include the wildcard characters or meaning any character and or meaning any substring including an empty substring
58. r Manual 4 19 4 3 10 Ov note Example 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 This gives a set of couples department and average of the salaries of the employees working in this department when this average is more than 30000 So the type of the result is bag lt struct department integer avg salary float In Os the syntax of partition attributes does not accept the keyword struct and thus is always given as a list of criteria separated by commas See Section 4 4 1 Order by Operator If select query is a select from where or a select from where group by query and if e1 e2 eare expressions then select query order by e eo e is an expression It returns a list of the selected elements sorted by the function e and inside each subset yieldingthesamee sorted by ep and the final subsub set sorted by en Example select p from Persons p order by p age p name This sorts the set of persons on their age then on their name and puts the sorted objects into the result as a list Each sort expression criterion can be followed by the keyword asc or desc specifying respectively an ascending or descending order The default order is that of the previous declaration For the first expression the default is ascending 4
59. r this description have already been defined 4 28 ODMG OQL User Manual Syntactical Abbreviations Structure Construction 4 4 1 Structure Construction The structure constructor was introduced in Section 4 3 4 2 Alternate syntax are allowed in two contexts select clause and group by clause In both contexts the SQL syntax is accepted along with the one already defined select projection projection select group by projection projection where projection is in one of the following forms i expression as identifier ii identifier expression iii expression This is an alternate syntax for struct identifier expression identifier expression If thereis only one projection and the syntax iii isused in a select clause then it is not interpreted as a structure construction but rather the expression stands as it is Furthermore a iii expression is only valid if it is possible to infer the name of the corresponding attribute the identifier This requires that the expression denotes a path expression possibly of length one ending in a property whose name is then chosen as the identifier Example select p name salary student id from Professors p p teaches This query returns a bag of structures bag lt struct name string salary float student id integer O gt note O accepts the 3 alternatives of the projection syntax in the select part as well as
60. rpose it uses the name of a class as a constructor Attributes of the object of this class can be initialized explicitly by any valid expression For instance to create a new building with 2 apartments if there is a type name in the schema called List apart defined by tydedef List lt lt Ref lt Apartment gt gt List apart the query is Building address Address number 10 Street Main street apartments List apart list Apartment number 1 Apartment number 2 Method invoking OQL allows method calls with or without parameters anywhere the result type of the method matches the expected type in the query In case the method has no parameter the syntax for method call is the same as for accessing an attribute or traversing a relationship If the method has parameters these are given between parenthesis This flexible syntax frees the user from knowing whether the property is ODMG OQL User Manual OQL by example Polymorphism stored an attribute or computed a method For instance to get the age of the oldest child of Paul we write the following query select max select c age from c in p children from p in Persons where p name Paul Of course a method can return a complex object or a collection and then its call can be embedded in a complex path expression For instance inside a building b to know who inhabits those least expensive apartment we use the following pa
61. rtable distributed high performance dynamic object oriented database system Database Engine E pee Ea Ea Database Engine e O5Store The database management system provides low level facilities through OsStore API to access and manage a database disk volumes files records indices and transactions e OsEngine The object database engine provides direct control of schemata classes objects and transactions through OpEngine API It provides full text indexing and search capabilities with OsSearch and spatial indexing and retrieval capabilities with OsSpatial It includes a Notification manager for informing other clients connected to the same O server that an event has occurred a Version manager for handling multiple object versions and a Replication API for synchronizing multiple copies of an O2 system 1 2 ODMG OQL User Manual System Overview Programming Languages Ov objects may be created and managed using the following programming languages utilizing all the features available with Os persistence collection management transaction management OQL queries etc C C Java OC OQL Os functions can be invoked by C programs ODMG compliant C binding ODMG compliant Java binding A powerful and elegant object oriented fourth generation language specialized for easy development of object database applications ODMG standard easy to
62. ry define identifier as query Basic see Section 4 3 3 query hil query true query false query integer literal query float literal query character literal query string literal query entry name query query name query bind argument query from variable name query query 1 A bind argument allows to bind expressions from a programming language to a query when embedded into this language see Chapters on language bindings 4 32 ODMG OQL User Manual OQL BNF Grammar 4 5 1 3 4 5 1 4 4 5 1 5 4 5 1 6 Simple Expression see Section 4 3 5 query query query query query query query query query query query query query query query query query query mod query abs query query query Comparison see Section 4 3 5 query query query comparison operator query query like string literal comparison operator comparison operator l comparison operator gt comparison operator lt comparison operator gt comparison operator lt Boolean Expression see Section 4 3 5 query query query not query query and query query Of query Constructor see Section 4 3 4 query query query query query query query query query type name query type name identifier query identifier query struct identifier query identifier query set query query bag qu
63. s from c in TheCompanies returns a set of clients ODMG OQL User Manual 2 21 2 GETTING STARTED 2 7 Combining operators OQL is a complete functional language in that every operator can be combined with any other operator You can use combine and build up operators universal and existential quantifiers wild card operators standard set operators as well as list concatenation ordering and grouping operators on sets bags and lists For example select cl name paid sum select p price from p in cl order from cl in Globe clients where count cl order gt 2 order by sum select p price from p in cl order This sorts all the clients with more than two orders by how much they have paid to the company 351 100000 Haddock 24 800000 f 2 8 Indexes When OQL extracts one or more elements from a collection using a specified predicate or order operation it must scan the whole collection to find the required elements You can improve performance if the system is able to directly access the matching elements This is done by establishing an index on a collection An index maps a key to one or more elements of a named collection 2 22 ODMG OQL User Manual Indexes Display index Whenever a program searches for elements of the collection using the key the system uses the index to quicken the search This entire process is
64. st salary the first in the list and take the corresponding street name This is the final result sorted salary map 0 street In a single query we could have written 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 ODMG OQL User Manual 4 OQL REFERENCE This chapter gives the full referencial information of the object query language OQL It is divided into the following sections Introduction Principles Language Definition e Syntactical Abbreviations OQL BNF The information given below is the same as that of the ODMG standard with notes added on how to use this language with Oo 1 The Object Database Standard ODMG 93 Atwood Duhl Ferran Loomis and Wade Edited by R G G Cattell 1996 Morgan Kaufman Publishers ODMG OQL User Manual 4 1 4 1 4 2 Introduction In this chapter a formal and complete definition of the language is given For each feature of the language we give the syntax its semantics and an example Alternate syntax for some features are described in Section 4 4 which completes OQL in order to accept any syntactical form of SQL The chapter ends with the formal syntax which is gi
65. th expression b less expensive is used by name Although 1ess expensive is a method we traverse it as if it were a relationship Polymorphism A major contribution of object technology is the possibility of manipulating polymorphic collections and thanks to the late binding mechanism to carry out generic actions on the elements of these collections For instance the set Persons contains objects of class Person Employee and Student So far all the queries against the Persons extent dealt with the three possible classes of objects of the collection A query is an expression whose operators operate on typed operands It is correct if thetype of operands matches thoserequired by the operators In this sense OQL is a typed query language This is a necessary condition for an efficient query optimizer When a polymorphic collection is filtered for instance Persons its elements are statically known to be of that class for instance Person This means that a property of a subclass attribute or method cannot be applied to such an element except in two important cases late binding to a method or explicit class indication Late binding To list the activities of each person weuse the following query select p activities from p in Persons ODMG OQL User Manual 3 11 OQL RATIONALE activities is a method which has 3 incarnations one for Student one for Employee and one for generic Person
66. the following query as select e from e in aSet order by 4 24 ODMG OQL User Manual Language Definition Conversion Expressions 4 3 13 3 4 3 13 4 Removing Duplicates If eis an expression whose value is a collection then distinct e is an expression whose value is the same collection after removing the duplicated elements If eis a bag distinct e is a set lf eis an ordered collection the relative ordering of the remaining elements is preserved Flattening Collection of Collections If eis a collection valued expression flatten e is an expression This converts a collection of collections of t into a collection of t So this flattening operates at the first level only Assuming the type of e to be col lt col lt t gt gt the result of flatten e is If col is a set resp a bag the union of all co12 t is done and the result is a set t resp bag t If col is alist resp an array and col is a list resp an array as well the concatenation of all col lt t gt is done following the order in col and the result is co12 t which is thus a list resp an array Of course duplicates if any are maintained by this operation If col is alist or an array and col is a set or a bag the lists or arrays are converted into sets the union of all these sets is done and the result is a set t therefore without duplicates Examples flatten list set 1 2 3 set 3 4 5 6
67. tions on average count sum maximum and minimum values of partitions You do this using the having clause ODMG OQL User Manual 2 17 GETTING STARTED For example if you wish to select only groups with more than one salary number count partition select salary from e in Globe employees group by e salary having count partition gt 1 The following screen is displayed EL sme 1 2 ODMG OQL User Manual Operators Like Order by The like operator allows you to test part of a character string The character stands for any string including the empty string The query select distinct e salary from e in Globe employees where e name like Sp returns the salaries of all employees whose names begin with Sp 1198 000000 B40 000000 You can obtain a sorted list using the order by clause For example to sort the employees by name and by age select e from e in Globe employees order by e name e age The result of an order by operation is always a list even though the source of the objects to sort the set employees in this case may bea set This query returns a list of employees their order is alphabetical by name and then by age ODMG OQL User Manual 2 19 2 GETTING STARTED 2 5 Set operators The standard set operations are defined on set and bag u
68. tive operators 3 12 Architecture Op 1 2 Arithmetic 4 33 Array 2 6 2 6 2 9 Constructing 4 8 Set conversion 2 21 array 2 11 2 12 3 9 Array value 2 6 Atomic value 2 5 Attribute 4 11 avg 2 13 3 12 4 15 B Bag 2 6 2 12 Constructing 4 8 bag 2 11 2 12 3 9 Boolean 4 33 Browser Interface 1 5 Unix 1 5 Windows NT 1 6 by 2 19 C C 1 3 C Interface 1 3 C binding 3 2 3 3 Class indicator 3 12 Collection 2 6 3 3 3 7 4 25 indexed expression 4 21 Named 2 22 Collection expression 4 34 Combining operators 2 12 2 22 Comparison 4 33 concatenation 4 22 Construction Array 3 9 Bag 3 9 List 3 9 Set 3 9 Struct 3 9 Constructor 2 11 3 9 4 33 Conversion 2 21 4 24 4 35 count 2 13 3 12 4 15 Creating objects 2 12 D Data manipulation 3 9 Database entry point 2 4 define 2 14 2 20 3 12 difference 4 23 distinct 2 7 3 7 ODMG OQL User Manual INDEX E element 2 14 4 24 except 2 20 3 12 4 23 Existential quantification 2 22 3 12 4 14 exists 2 15 3 12 F first 4 22 flatten 2 21 Flattening 4 25 forall in 3 12 from 3 8 G group by 2 15 3 12 3 13 4 18 H Hypertext links 1 6 intersect 2 20 3 12 4 23 intersection 4 23 Java 1 3 Join 3 8 Join query 2 8 last 4 22 Late binding 3 11 like 2 19 List 2 6 2 6 2 9 Constructing 4 7 Set conversion 2 21 Values 2 6 list 2 11 3 9 4 24 listtoset 2 21 M max 2 13 3 12 4 15 Membership 4 15 Method call
69. totally transparent to you as the programmer The absence or presence of an index has no effect on program code only on system performance The benefits of indexes include the following Complete logical and physical independence You do not have to change your query to use indexing Indexes are created by administration commands e High performance during use and maintenance Access from an index means constant time access irregardless of the size of the collection Example Defining an index for all employees create index the employees on salary e The following query will then be optimized select e from e in the employees where e salary 2 1000 and e salary lt 5000 Display index The display index query allows you to see how OQL will use existing indexes in que ries you will make To stop this feature execute display index again Note Please refer to the System Administration Guide for details on how to create and manage indexes ODMG OQL User Manual 2 23 2 GETTING STARTED 2 9 Chapter Summary This chapter has covered the following points Basic queries To query any database you need various entry points In Os these are the named instances i e named objects and named values Simple queries include calling an entry point applying a method to a named object extracting a field etc Select from where The select from whereclaus
70. tract any of its elements if you know their position For example you can extract the first element of the list in clients as follows Globe clients 0 In OQL you count list elements from 0 Haddock For OQL an array behaves the same way as a list Call of a method To apply a method to an object is a base query e g Globe title This applies the method title to the object Globe and returns the result of the method title 2 2 Select from where The select from where clause enables you to extract those elements meeting a specific condition from a collection Os collections include set bag a multi set or set with duplicates list an insertable and dynamic array or array The OQL query has the following structure select defines the structure of the query result from introduces the collections against which the query runs 2 6 ODMG OQL User Manual Select from where Set Set where introduces a predicate that filters the collection This section now describes how to use this clause A set is a non ordered collection The most frequent query on a set is a filter This consists of extracting the elements of a set which have certain characteristics For example select e from e in Globe employees where e salary gt 200 00 This query returns those employees working at the International Globe with a salary over 200
71. use SQL like object query language with special features for dealing with complex O gt objects and methods O2 Development Tools Os5Graph O sLook OoKit OpsTools Create modify and edit any type of object graph Design and develop graphical user interfaces provides interactive manipulation of complex and multimedia objects Library of predefined classes and methods for faster development of user applications Complete graphical programming environment to design and develop O database applications Standard Development Tools All standard programming languages can be used with standard environments e g Visual C Sun Sparcworks External Interfaces OsCorba O DBAccess O20DBC O5Web Create an O Orbix server to access an O database with CORBA Connect O applications to relational databases on remote hosts and invoke SQL statements Connect remote ODBC client applications to Oo databases Create an Oo World Wide Web server to access an Os database through the internet network ODMG OQL User Manual 1 3 INTRODUCTION OQL OQL is an object oriented SQL like query language OQL is the query language of the ODMG 93 standard It can be used in two different ways either as an embedded function in a programming language or as an ad hoc query language You can use OQL as a function called from O5C C C Smalltalk or Java in order to manipulate complex
72. values and methods Each construct produces a result which can then be used directly in the programming language Methods can be triggered to modify the database You will find that programming is easier because OQL can filter values using complex predicates whose evaluations are optimized by the OQL optimizer in Oo OQL can also be used interactively as an ad hoc query language allowing database queries from both technical and non technical users Interactive features include fast and simple browsing of the database 1 The Object Database Standard ODMG 93 Atwood Barry Duhl Eastman Ferran Jordan Loomis and Wade Edited by R G G Cattell 1996 Morgan Kaufman Publish ers 1 4 ODMG OQL User Manual System Overview Browser Interface Browser Interface The browser interface you see depends on the operating system you are using Unix In Unix the OsLook graphical user interface generator is used to generate the graphical form of OQL query results Figure 1 2 shows a typical query result in graphical form as generated by OsLook Eraser button EG236A Set collar 100mm gt E77 A134 Fixing element SL2SB6FGH iBnehoring support d P s Ros 788 n Masher KUNY 865 Washer gt Object icons CLog 2354 Clip KUNY 1348 lt Clip gt SELFX15 Fixing strap P865 Anchoring clip VOT DD4 Screw KUNY135 98 iScrew J7990 Joint KUNY23456 Joint ROS8 253 Washer KUNY2
73. ven in Section 4 5 Principles Our design is based on the following principles and assumptions OQL relies on the ODMG object model OQL is a superset of the standard SQL part which allows you to query a database Thus any select SQL sentence which runs on relational tables works with the same syntax and semantics on collections of ODMG objects Extensions concern Object Oriented notions like complex objects object identity path expression polymorphism operation invocation late binding etc OQL provides high level primitives to deal with sets of objects but does not restrict its attention to this collection construct Thus it also provides primitives to deal with structures lists arrays and treats all such constructs with the same efficiency OQL is a functional language where operators can freely be composed as soon as the operands respect the type system This is a consequence of the fact that the result of any query has a type which belongs to the ODMG type model and thus can be queried again OQL is not computationally complete It is an easy to use query language which provides easy access to an object database Based on the same type system OQL can be invoked directly from within programming languages for which an ODMG binding is defined e g C and SmallTalk Conversely OQL can invoke operations programmed in these languages OQL does not provide explicit update operators but rather can invoke operations

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