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Object Relational Database Systems:

1. Introduction

2. Objects in SQL3

3. Comparison of ODL/OQL and SQL3 Approaches

CIS 671

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Object Query Language (OQL)• “Bring the best of SQL to the object world.”• Syntax similar to SQL• Plus additional features• Works with programming languages where

ODMG has defined bindings– Java, C++, Smalltalk– Returns object matching type system of that language– May implement class operations in these languages

• SQL3 – “Bring the best of the object-oriented world to the relational world.”

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Overview: Object Relational DB SystemsSQL3

• Interfacing SQL to a programming language.– Cursors

• Data Types– Built-in– User Defined

• Operations

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Data Types – Built-in• smallint – 16-bit integer• integer – 32-bit integer• decimal(p,s) – decimal

number, precision p, scale s• float – 32-bit float• double – 64-bit float

• date – year, month, day• time – hour, minute, second• timestamp - year, month,

day, hour, minute, second, microsecond

• interval – year/month or day/time

• char(n) – fixed length, n ≤ 254• varchar(n) – varying length

– n ≤ 254 – 254 ≤ n ≤ 4000; no group by, etc.

• bit(n)• varbit(n)• boolean [SQL3]• Large Objects [SQL3]

– blob(binary large object)• ≤ 231 –1 bytes

– clob(character large object)• ≤ 231 –1 bytes

Domaine.g., create domain

SSN_TYPE as char(9)

not used much.

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Objects in SQL3

• Row objects, tuples or structs, and

• Abstract data types (ADT), general objects, used as components of tuples.

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Moviestitleyearlength /* in minutes */filmType:{color,

blackAndWhite}lengthInHoursstarNamesotherMovies

Movies Example: Movies, Stars, StudiosODL Version

Starsnameaddress

streetcity

Studiosname

stars starredIn

ownedBy owns

Set of stars of this movie.

Other movies by star of this movie.

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Movies Example: Movies, Stars, StudiosER Version

Movie MovieStarStarsInN M

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Row Types [SQL3 or SQL:1999]• Row-type Definition:

create row type T (<component-declaration-list>)

• Examples:create row type AddressType(

street char(50),city char(20));

create row type StarType(name char(30),address AddressType

);

create table MovieStar of type StarType;

select MovieStar..name, MovieStar..address..street

from MovieStarWhere MovieStar..address..city = ‘Columbus’;

Cannot directly represent set of

movies starred-in.

Double dotPath Exp.

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References • SQL3’s way to represent “objects”.

create row type StarType(name char(30),address AddressType );

create row type MovieType(title char(30),year integer );

create row type StarsInType(star ref(StarType),movie ref(MovieType) );

create table Movie of type MovieType;create table MovieStar of type StarType;create table StarsIn of type StarsInType;

select movie -> titlefrom StarsInwhere star->name = ‘Mel Gibson’

Dereferencing operator (->)

Movie MovieStarStarsInN M

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Object Identifiers• May be explicitly declared and accessed.• Can serve as both object ID and primary key.

create row type MovieType(movie_id ref(MovieType),title char(30),year integer );

create row type StarType(star_id ref(StarType),name char(30),address AddressType );

create table Movie of type MovieTypevalues for movie_id are system generated;

create table MovieStar of type StarTypevalues for star_id are system generated;

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Specifying Scope to Aid Performancecreate row type StarsInType(

star ref(StarType),movie ref(MovieType) );

create table StarsIn of type StarsInTypescope for star is MovieStar,scope for movie is Movie;

select movie -> titlefrom StarsInwhere star->name = ‘Mel Gibson’

How to find movies?Examine every StarsIn tuple.

Index on name attribute.

Could have another relation with StarType.

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Objects in SQL3

• Row objects, tuples or structs (REVIEW), and

• Abstract data types (ADT), general objects, used as components of tuples.

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Abstract data type (ADT)

• Used as components of tuples, – Not as tuples themselves.

• Have tuple structure.

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ADT - “Built-in” Functions• Constructor functions returning new object of type.

– All attributes initially null.– If T is name of ADT, then T() is constructor.

• Observer functions for each attribute.– If A is attribute name & X is variable whose value is an

object of the ADT, then A(X) (or X.A) is the value of attribute A of object X.

• Mutator functions for each attribute.– Sets the value of that attribute to a new value.– Normally used on left side of assignment

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Example: Address ADT

create type AddressADT (street char(50),city char(20),equals addrEq,less than addrLT

other functions could be declared here);

Addresses will be encapsulated• Access to street & city allowed only if observer

and mutator functions made public.• Functions addrEQ and addrLT defined later.

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More interesting example: ADT for Mpegs

create type Mpeg (video blob,length integer,copyright varchar(255),equals default, /* i.e., identity */less than none

definitions of MPEG functions go here);

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Defining Methods for ADT’s

• Function types– Internal

• Written in SQL.

– External• Written in C++, Java, etc.

• Only signature appears in definition of the ADT.

Function <name> ( <arguments> ) returns <type> ;

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Extended SQL • := used as assignment operator.• Variable local to the function can be declared by

giving its name, preceded by a colon and followed by its type.

:schar(50) /* s will be a street */

• Dot operator used to access components of a structure:a.street := :s /* a, an address */

• Boolean values can be expressed as in where clauses.

• begin and end are used to collect several statements into the body of a function.

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Some Functions for the ADT AddressADT:1. Constructor Function

function AddressADT (:s char(50), :c char(20))returns AddressADT;

:a AddressADT; /* declare local variable */

begin:a := AddressADT(); /* use built-in constructor */

:a.street := :s;:a.city := :c;return :a;

end;

Note: We can use the same name, AddressADT, as the default constructor.

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More Functions for the ADT AddressADT:

2. equals (addrEq )and less than (addrLT) Functions

function addrEq (:a1 AddressADT, :a2 AddressADT)returns boolean;

return ( :a1.street = :a2.street and:a1.city = :a2.city);

function addrLT (:a1 AddressADT, :a2 AddressADT)returns boolean;

return ( (:a1.city < :a2.city) or(:a1.city = :a2.city and :a1.street < :a2.street));

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Another Function for the ADT AddressADT:

function fullAddr (:a AddressADT)returns char(82);

:z char(10); /* for the zip code nnnnn-nnnn*/

begin:z := findZip(:a.street, :a.city );return (:a.street || ‘ ‘ || :a.city || ‘ ‘ || :z);

end;

3. fullAddr, function returning the full address, including zip code, as a single character string

findZip, an externally defined function which looks up the zip code for a given city and street.

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External Functions• ADT Definition must include:

– Signature.– Specification of programming language in

which function is written.

declare external findZipchar(50), char(20) returns char(10)language Java;

Arguments passed according to Java conventions.

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Comparison of ODL/OQL and SQL3 Approaches

• Similarities outweigh differences, even though origins different:– ODL/OQL: object-oriented programming languages.– SQL3: relational database languages.

• Have effectively adopted ideas from each other.

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Comparing ODL/OQL vs. SQL3actually

ODL/OQL vs. SQL3 row types vs. SQL3 ADT types

1. Programming environment.

2. Role of relations.

3. Encapsulation.

4. Extents of classes.

5. Mutability of objects.

6. Object identity.

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ODL/OQL vs. SQL3 row types vs. SQL3 ADT types

1. Programming environment

• OQL– Assumes statements embedded in OO programming

language, C++, Java, …

• SQL3– Objects not objects of surrounding programming

language.– External functions in SQL3 ADT’s provide

additional flexibility.

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ODL/OQL vs. SQL3 row types vs. SQL3 ADT types

2. Role of relations

• OQL– Sets and bags of objects or structures are central.

• SQL3– Relations are central.– Row types describe relations.– ADT’s describe new types for attributes.

• Collections of structures in ODL/OQL similar to Relations in SQL3.

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• SQL3 Row Types– Not encapsulated.

– Querying & modifying relations, tuples, and components allowed.

• SQL3 ADT’s– Encapsulated in the usual sense.

• ODL Classes– Similar to SQL3 ADT’s in encapsulation

ODL/OQL vs. SQL3 row types vs. SQL3 ADT types

3. Encapsulation

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ODL/OQL vs. SQL3 row types vs. SQL3 ADT types

4. Extents for Classes

• OQL– Single extent maintained for each class.

– Thus references (relationships in OQL) always refer to some member or members of this extent.

• SQL3– An extent for a row type allowed, but not required.

• If no extent for a row type, then may be problem finding relation containing referenced tuple.

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ODL/OQL vs. SQL3 row types vs. SQL3 ADT types

5. Mutability of Objects

• Immutable Object: once created, no part of its values can change.– Objects of elementary type, e.g., integers or strings, are immutable.

• Mutable Object: components may change, while object retains its identity.

• ODL Classes & SQL3 Row Types– Define classes of mutable objects.– ODL/OQL: modification occurs through surrounding programming

language, not through OQL.

• SQL3 ADT’s– Mutator functions applied to their values result in new value, which may

replace old one.• Similar to SQL update statement on integer-valued attribute produces a new

integer that might replace the old integer in the tuple.

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ODL/OQL vs. SQL3 row types vs. SQL3 ADT types

6. Object Identity

• OQL & SQL3 ADT’s– “Standard” OID: system generated, which

cannot be stored or manipulated by user.

• SQL3 Row Type– User can create “primary key”.– Without this relations would usually have two

“keys”:• OID• Surrogate value, e.g., Employee_ID.