Chapter12_accessible.pptx

Fundamentals of Database Systems

Seventh Edition

Chapter 12

Object and Object-Relational Databases

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Learning Objectives

12.1 Overview of Object Database Concepts

12.2 Object-Relational Features

12.3 Object Database Extensions to S Q L

12.4 O D M G Object Model and the Object Definition Language O D L

12.5 Object Database Conceptual Design

12.6 The Object Query Language O Q L

12.7 Overview of the C + + Language Binding

Object and Object-Relational Databases

Object databases (O D B)

Object data management systems (O D M S)

Meet some of the needs of more complex applications

Specify:

Structure of complex objects

Operations that can be applied to these objects

Overview of Object Database Concepts (1 of 2)

Introduction to object-oriented concepts and features

Origins in O O programming languages

Object has two components:

State (value) and behavior (operations)

Instance variables (attributes)

Hold values that define internal state of object

Operation is defined in two parts:

Signature (interface) and implementation (method)

Overview of Object Database Concepts (2 of 2)

Inheritance

Permits specification of new types or classes that inherit much of their structure and/or operations from previously defined types or classes

Operator overloading

Operation’s ability to be applied to different types of objects

Operation name may refer to several distinct implementations

Object Identity, and Objects Versus Literals

Object has Unique identity

Implemented via a unique, system-generated object identifier (O I D)

Immutable

Most O O database systems allow for the representation of both objects and literals (simple or complex values)

Complex Type Structures for Objects and Literals (1 of 2)

Structure of arbitrary complexity

Contain all necessary information that describes object or literal

Nesting type constructors

Generate complex type from other types

Type constructors (type generators):

Atom (basic data type – int, string, etc.)

Struct (or tuple)

Collection

Complex Type Structures for Objects and Literals (2 of 2)

Collection types:

Set

Bag

List

Array

Dictionary

Object definition language (O D L)

Used to define object types for a particular database application

Figure 12.1 Specifying the Object Types EMPLOYEE, DATE, and DEPARTMENT Using Type Constructors

Figure 12.2 Adding Operations to the Definitions of EMPLOYEE and DEPARTMENT

Encapsulation of Operations (1 of 2)

Encapsulation

Related to abstract data types

Define behavior of a class of object based on operations that can be externally applied

External users only aware of interface of the operations

Can divide structure of object into visible and hidden attributes

Encapsulation of Operations (2 of 2)

Constructor operation

Used to create a new object

Destructor operation

Used to destroy (delete) an object

Modifier operations

Modify the state of an object

Retrieve operation

Dot notation to apply operations to object

Persistence of Objects

Transient objects

Exist in executing program

Disappear once program terminates

Persistent objects

Stored in database, persist after program termination

Naming mechanism: object assigned a unique name in object base, user finds object by its name

Reachability: object referenced from other persistent objects, object located through references

Figure 12.3 Creating Persistent Objects by Naming and Reachability

Type (Class) Hierarchies and Inheritance (1 of 3)

Inheritance

Definition of new types based on other predefined types

Leads to type (or class) hierarchy

Type: type name and list of visible (public) functions (attributes or operations)

Format:

Type (Class) Hierarchies and Inheritance (2 of 3)

Subtype

Useful when creating a new type that is similar but not identical to an already defined type

Subtype inherits functions

Additional (local or specific) functions in subtype

Example:

EMPLOYEE subtype-of PERSON: Salary, Hire_date, Seniority

STUDENT subtype-of PERSON: Major, Gpa

Type (Class) Hierarchies and Inheritance (3 of 3)

Extent

A named persistent object to hold collection of all persistent objects for a class

Persistent collection

Stored permanently in the database

Transient collection

Exists temporarily during the execution of a program (e.g. query result)

Other Object-Oriented Concepts

Polymorphism of operations

Also known as operator overloading

Allows same operator name or symbol to be bound to two or more different implementations

Type of objects determines which operator is applied

Multiple inheritance

Subtype inherits functions (attributes and operations) of more than one supertype

Summary of Object Database Concepts

Object identity

Type constructors (type generators)

Encapsulation of operations

Programming language compatibility

Type (class) hierarchies and inheritance

Extents

Polymorphism and operator overloading

Object-Relational Features: Object D B Extensions to S Q L

Type constructors (generators)

Specify complex types using U D T

Mechanism for specifying object identity

Encapsulation of operations

Provided through user-defined types (U D T s)

Inheritance mechanisms

Provided using keyword UNDER

User-Defined Types (U D Ts) and Complex Structures for Objects (1 of 2)

U D T syntax:

Can be used to create a complex type for an attribute (similar to struct – no operations)

Or: can be used to create a type as a basis for a table of objects (similar to class – can have operations)

User-Defined Types (U D Ts) and Complex Structures for Objects (2 of 2)

Array type – to specify collections

Reference array elements using []

CARDINALITY function

Return the current number of elements in an array

Early S Q L had only array for collections

Later versions of S Q L added other collection types (set, list, bag, array, etc.)

Object Identifiers Using Reference Types

Reference type

Create unique object identifiers (O I D s)

Can specify system-generated object identifiers

Alternatively can use primary key as O I D as in traditional relational model

Examples:

REF IS SYSTEM GENERATED

Creating Tables Based on the U D T s

INSTANTIABLE

Specify that U D T is instantiable

The user can then create one or more tables based on the U D T

If keyword INSTANTIABLE is left out, can use U D T only as attribute data type – not as a basis for a table of objects

Encapsulation of Operations

User-defined type

Specify methods (or operations) in addition to the attributes

Format:

Figure 12.4a Illustrating Some of the Object Features of S Q L. Using U D T S as Types for Attributes Such as Address and Phone

Figure 12.4b Illustrating Some of the Object Features of S Q L. Specifying U D T for PERSON_TYPE

Specifying Type Inheritance (1 of 2)

NOT FINAL:

The keyword NOT FINAL indicates that subtypes can be created for that type

UNDER

The keyword UNDER is used to create a subtype

Figure 12.4c Illustrating Some of the Object Features of S Q L. Specifying U D Ts for STUDENT_TYPE and EMPLOYEE_TYPE as Two Subtypes of PERSON_TYPE (1 of 2)

Figure 12.4c Illustrating Some of the Object Features of S Q L. Specifying U D Ts for STUDENT_TYPE and EMPLOYEE_TYPE as Two Subtypes of PERSON_TYPE (2 of 2)

Specifying Type Inheritance (2 of 2)

Type inheritance rules:

All attributes/operations are inherited

Order of supertypes in UNDER clause determines inheritance hierarchy

Instance (object) of a subtype can be used in every context in which a supertype instance used

Subtype can redefine any function defined in supertype

Creating Tables Based on U D T

UDT must be INSTANTIABLE

One or more tables can be created

Table inheritance:

UNDER keyword can also be used to specify supertable/subtable inheritance

Objects in subtable must be a subset of the objects in the supertable

Figure 12.4d Illustrating Some of the Object Features of S Q L. Creating Tables Based on Some of the U D Ts, and Illustrating Table Inheritance

Specifying Relationships via Reference

Component attribute of one tuple may be a reference to a tuple of another table

Specified using keyword R E F

Keyword SCOPE

Specify name of table whose tuples referenced

Dot notation

Build path expressions

Used for dereferencing

Figure 12.4e Illustrating Some of the Object Features of S Q L. Specifying Relationships Using R E F and SCOPE

Summary of S Q L Object Extensions

U D T to specify complex types

INSTANTIABLE specifies if U D T can be used to create tables; NOT FINAL specifies if U D T can be inherited by a subtype

R E F for specifying object identity and inter-object references

Encapsulation of operations in U D T

Keyword UNDER to specify type inheritance and table inheritance

O D M G Object Model and Object Definition Language O D L (1 of 2)

O D M G object model

Data model for object definition language (O D L) and object query language (O Q L)

Objects and Literals

Basic building blocks of the object model

Object has five aspects:

Identifier, name, lifetime, structure, and creation

Literal

Value that does not have an object identifier

O D M G Object Model and Object Definition Language O D L (2 of 2)

Behavior refers to operations

State refers to properties (attributes)

Interface

Specifies only behavior of an object type

Typically noninstantiable

Class

Specifies both state (attributes) and behavior (operations) of an object type

Instantiable

Inheritance in the Object Model of O D M G

Behavior inheritance

Also known as I S – A or interface inheritance

Specified by the colon (:) notation

EXTENDS inheritance

Specified by keyword extends

Inherit both state and behavior strictly among classes

Multiple inheritance via extends not permitted

Built-In Interfaces and Classes in the Object Model

Collection objects

Inherit the basic Collection interface

Creates an iterator object for the collection

To loop over each object in a collection

Collection objects further specialized into:

set, list, bag, array, and dictionary

Figure 12.6 Inheritance Hierarchy for the Built-In Interfaces of the Object Model

Atomic (User-Defined) Objects

Specified using keyword class in O D L

Attribute

Property; describes data in an object

Relationship

Specifies inter-object references

Keyword inverse

Single conceptual relationship in inverse directions

Operation signature:

Operation name, argument types, return value

Figure 12.7 The Attributes, Relationships, and Operations in a Class Definition

Extents, Keys, and Factory Objects

Extent

A persistent named collection object that contains all persistent objects of class

Key

One or more properties whose values are unique for each object in extent of a class

Factory object

Used to generate or create individual objects via its operations

Object Definition Language O D L

Support semantic constructs of O D M G object model

Independent of any particular programming language

Example on next slides of a UNIVERSITY database

Graphical diagrammatic notation is a variation of E E R diagrams

Figure 12.9a An Example of a Database Schema. Graphical Notation for Representing O D L Schemas

Figure 12.9b An Example of a Database Schema. A Graphical Object Database Schema for Part of the UNIVERSITY Database (GRADE and DEGREE Classes Are Not Shown)

Figure 12.10 Possible O D L Schema for the UNIVERSITY Database in Figure 12.8(b) (1 of 2)

Figure 12.10 Possible O D L Schema for the UNIVERSITY Database in Figure 12.8(b) (2 of 2)

Interface Inheritance in O D L

Next example illustrates interface inheritance in O D L

Figure 12.11a An Illustration of Interface Inheritance Via “:”. Graphical Schema Representation

Figure 12.11b An Illustration of Interface Inheritance Via “:”. Corresponding Interface and Class Definitions in O D L

Object Database Conceptual Design

Differences between conceptual design of O D B and R D B, handling of:

Relationships

Inheritance

Philosophical difference between relational model and object model of data

In terms of behavioral specification

Mapping an E E R Schema to an O D B Schema (1 of 2)

Create O D L class for each E E R entity type

Add relationship properties for each binary relationship

Include appropriate operations for each class

O D L class that corresponds to a subclass in the E E R schema

Inherits type and methods of its superclass in O D L schema

Mapping an E E R Schema to an O D B Schema (2 of 2)

Weak entity types

Mapped same as regular entity types

Categories (union types)

Difficult to map to O D L

An n-ary relationship with degree n > 2

Map into a separate class, with appropriate references to each participating class

The Object Query Language O Q L

Query language proposed for O D M G object model

Simple O Q L queries, database entry points, and iterator variables

Entry point: named persistent object

Iterator variable: define whenever a collection is referenced in an O Q L query

Query Results and Path Expressions

Result of a query

Any type that can be expressed in O D M G object model

O Q L orthogonal with respect to specifying path expressions

Attributes, relationships, and operation names (methods) can be used interchangeably within the path expressions

Other Features of O Q L (1 of 2)

Named query

Specify identifier of named query

O Q L query will return collection as its result

If user requires that a query only return a single element use element operator

Aggregate operators

Membership and quantification over a collection

Other Features of O Q L (2 of 2)

Special operations for ordered collections

Group by clause in O Q L

Similar to the corresponding clause in S Q L

Provides explicit reference to the collection of objects within each group or partition

Having clause

Used to filter partitioned sets

Overview of the C + + Language Binding in the O D M G Standard

Specifies how O D L constructs are mapped to C + + constructs

Uses prefix d_ for class declarations that deal with database concepts

Template classes

Specified in library binding

Overloads operation new so that it can be used to create either persistent or transient objects

Summary

Overview of concepts utilized in object databases

Object identity and identifiers; encapsulation of operations; inheritance; complex structure of objects through nesting of type constructors; and how objects are made persistent

Description of the O D M G object model and object query language (O Q L)

Overview of the C++ language binding

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