2/1/04database systems: salman azhar1 object-relational databases salman azhar user-defined types...

2/1/04 Database Systems: Salman Azhar 1

Object-Relational DatabasesSalman Azhar

User-Defined TypesObject IDs

Nested TablesThese slides use some figures, definitions, and explanations from

Elmasri-Navathe’s Fundamentals of Database Systemsand Molina-Ullman-Widom’s Database Systems


Merging Relational and Object Models

Object-oriented models support interesting data types --- not just flat

files.• E.g., Maps, multimedia, etc.

The relational model supports very-high-level queries

Object-relational databases are an attempt to get the best of both.


Evolution of DBMSs

Object-oriented DBMSs fell short because they did not offer the

efficiencies of a relational DBMS. Object-relational extensions to

relational DBMSs capture much of the advantages of OO yet retain the relation as the

fundamental abstraction.


SQL-99 and DBMS Features

SQL-99 includes many of the object-relational features to be described.

However, being so new, different DBMSs use different approaches. We’ll sometimes use features and

syntax from Oracle and SQL Server.


User Defined Types

A user-defined type, or UDT, is essentially a class definition, with a structure and methods.

Two uses:1. As a row-type, that is, the type of a

relation.2. As an column-type, that is, the type

if attribute in a relation.


UDT Definition

CREATE TYPE <typename> AS (<list of elements, as in CREATE TABLE>

); Oracle syntax:

1. Add “OBJECT” as inCREATE TYPE <name> AS OBJECT.

2. Follow with / to have the type stored.


Example: UDT Definition

CREATE TYPE DealerType AS (

name CHAR(20),

addrCHAR(20)

);

CREATE TYPE CarType AS (

nameCHAR(20),

manfCHAR(20)

);


References

If T is a type, then REF T is the type of a reference to T this means a pointer to an object of

type T. often called an “object ID” in OO

systems. Unlike object ID’s, a REF is visible,

although it is usually gibberish.


Example: REFCREATE TYPE SellsType AS (

dealer REF DealerType,

car REF CarType,

price FLOAT

); SellsType objects look like: REF DealerType REF CarType FLOAT

30000

Reference to aDealerType

objectReference to aCarType object


UDTs as Row Types

A table may be defined to have a schema that is a row type, rather than by listing its elements.

Syntax: CREATE TABLE <table name> OF <type name>;

Creates a table where each row is of <type name>


Example: Creating Tables

CREATE TABLE Dealer OF DealerType;

CREATE TABLE Car OF CarType;

CREATE TABLE Sells OF SellsType;


Values of Relations with a Row-type

Technically, a relation declared to have a rowtype DealerType (such as Dealer), is not a set of pairs it is a unary relation

• whose tuples are objects with two components:

– name and addr.


Type Constructors

Each UDT has a type constructor of the same name that wraps objects of that type.


Example: Type Constructor

The querySELECT * FROM Dealer;

Produces “tuples” such as:DealerType(‘AutoNation’, ‘Maple St.’)


Aliasing:Accessing Values From a Rowtype

In Oracle and MS SQL Server, the dot works as expected but it may not work in all DBMS so it is a good idea, to use an alias for

every relation, when O-R features are used.

Example:SELECT dd.name, dd.addrFROM Dealer dd;


Accessing Values: SQL-99 Approach

In SQL-99, each attribute of a UDT has: Get method

• Called Generator methods (get the value) and Set method

• Called Mutator methods (change the value)

These methods have the same name as the attribute. The generator for A takes no argument, as

A( ). The mutator for A takes a new value as

argument, as A(v).


Example: SQL-99 Value Access

ConsiderSELECT dd.name, dd.addr

FROM Dealer dd;

The same query in SQL-99 isSELECT dd.name( ), dd.addr( )

FROM Dealer dd;

Explicitly access

generators


Inserting Row Type Values(Oracle Style)

We can use a standard INSERT statement, remembering that a relation with a row type

is really unary and needs that type constructor.

Example:INSERT INTO Dealer VALUES(

DealerType(‘AutoNation’, ‘Maple St.’)

);

Note: DealerType is a constructor


Inserting Values: SQL-99 Style

1. Create a variable X of the suitable type,

using the constructor method for that type.

2. Use the mutator methods for the attributes

to set the values of the fields of X.

3. Insert X into the relation.


Example: SQL-99 Insert

The following must be part of a procedure, so we have a variable newDealer.

SET newDealer = DealerType( );

newDealer.name(‘AutoNation’);newDealer.addr(‘Maple St.’);

INSERT INTO Dealer VALUES(newDealer);

Mutatormethodschange

newDealer’sname and addrcomponents.

Construct anew dealer

Insert newvalues


UDTs as Column Types

A UDT can be the type of a column (an attribute) in either another UDT definition or in a CREATE TABLE statement

Use the name of the UDT as the type of the attribute


Example: Column Type

CREATE TYPE AddrType AS (street CHAR(30),city CHAR(20),zip INT

);CREATE TABLE Buyers (name CHAR(30),addr AddrType,favCar CarType

);

Values of addr andfavCar components

are objects with 3 and2 fields, respectively.


Oracle Problem With Field Access

You can access a field F of an object that is the value of an attribute A by A.F .

However, you must use an alias, say tt, for the table T with attribute A, as tt.A.F .


Example: Field Access in Oracle

Wrong (can’t have nothing):SELECT favCar.nameFROM Buyers;

Wrong (can’t have the table name):SELECT Buyers.favCar.nameFROM Buyers;

Right (must have alias name):SELECT b.favCar.nameFROM Buyers b;


Following References (REFs)

A B denotes the value of the B component of the object pointed to by A

A B makes sense if:1. A is of type REF T2. B is an attribute (component) of

objects of type T


Following REF’s: Oracle Style

REF-following is implicit in the dot Just follow a REF by a dot and a

field of the object referred to Example:

SELECT ss.car.nameFROM Sells ssWHERE ss.dealer.name =

‘AutoNation’;

Field name inobject car intable aliases

as ss


Oracle’s DEREF Operator -- Motivation

If we want the set of car objects for the cars sold by AutoNAtion, we might try:

SELECT ss.carFROM Sells ssWHERE ss.dealer.name = ‘AutoNation’;

Legal, but ss.car is a REF, hence gibberish! Need to add DEREF

object car (ref) in

table aliases as ss


Using DEREF

To see the CarType objects, use:SELECT DEREF(ss.car)

FROM Sells ss

WHERE ss.dealer.name = ‘AutoNation’;

Produces values like:CarType(‘MiniCooper’, ‘BMW’)


Methods Classes are more than structures;

they may have methods. Declare in CREATE TYPE Define methods in a CREATE TYPE

BODY statement Use T-SQL syntax for methods.

• (T-SQL: MS SQL Server = PL/SQL: Oracle)

Variable SELF refers to the object to which the method is applied (this)


Method Definition

Form of create-body statement:CREATE TYPE BODY <type name> AS

PROCEDUREmethodName(arg <ARGTYPE>)

RETURN <RETURNTYPE>BEGIN…END;

END;


Method Definition – Oracle Style

Method definitions are called PL/SQL procedure definitions in Oracle

Oracle uses “MEMBER FUNCTION” in place of “PROCEDURE”


Example: Method Definition

CREATE TYPE BODY SellsType AS

MEMBER FUNCTION

priceConvert(rate FLOAT) RETURN FLOAT IS

BEGIN

RETURN rate * SELF.price;

END;

END;

Argument & argument type

Sorta like “this”

keyword in C++/Java

(parenthesis only if an

argument is passed)


Method Use

objectName.methodName(arguments if any)

Example:SELECT ss.car.name, ss.priceConvert(1.33)

FROM Sells ss

WHERE ss.dealer.name = ‘AutoNation’;


Example: Nested Table Type

CREATE TYPE CarType AS OBJECT (name CHAR(20),edition CHAR(20)

);GOCREATE TYPE CarTableType AS TABLE OF CarType;

GO


Example --- Continued

Use CarTableType in a Manfs relation that stores the set of cars by each manufacturer in one tuple for that manufacturer.

CREATE TABLE Manfs (

name CHAR(30),

addr CHAR(50),

car carTableType

);


Nested Tables

Allows values of row components to be whole relations

If T is a UDT, we can create a type S whose values

are relations with rowtype T, by:

CREATE TYPE S AS TABLE OF T ;


Querying a Nested Table

We can print the value of a nested table like any other value

But these values have two type constructors:

1. For the table2. For the type of tuples in the table


Example: Query a Nested Table

Find the cars by Toyota:SELECT car FROM ManfsWHERE name = ‘Toyota’;

Produces one value:CarTableType(CarType(‘Camry’, ‘LE’)CarType(‘Corolla’, ‘L’)….

)


Querying Within a Nested Table

A nested table can be converted to an ordinary relation by applying THE(…)

This relation can be used in FROM clauses like any other relation


Example: Use of THE

Find the cars made by Toyota:SELECT dd.edition

FROM THE(

SELECT car FROM Manfs

WHERE name = ‘Toyota’;

) dd

WHERE dd.name = ‘Camry’;

2/1/04database systems: salman azhar1 object-relational databases salman azhar user-defined types...

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