ITEC 3220 A Using and Designing Database Systems

ITEC 3220 A Using and Designing Database Systems Instructor: Gordon Turpin Course Website: www. cse. yorku. ca/~gordon/itec 3220 S 07 Office: CSEB 3020

Appendix G Object-Oriented Database

Object Orientation • Object Orientation – Set of design and development principles – Based on autonomous computer structures known as objects • OO Contribution areas – – – Programming Languages Graphical User Interfaces Databases Design Operating Systems 3

Evolution of OO Concepts • Concepts stem from object-oriented programming languages Ada, ALGOL, LISP, SIMULA • OOPLs goals – – Easy-to-use development environment Powerful modeling tools for development Decrease in development time Make reusable code • OO Attributes – Data set not passive – Data and procedures bound together – Objects can act on self 4

OO Concepts: Objects • Abstract representation of a real-world entity – Unique identity – Embedded properties – Ability to interact with other objects and self • OID – Unique to that object – Assigned by system at moment of object’s creation – Cannot be changed under any circumstances – Can be deleted only if the object is deleted – Can never be reused 5

Attributes (Instance Variables) • Attributes: – Known as instance variables in OO environment • Domain: – Logically groups and describes the set of all possible values that an attribute can have 6

Object State • Set of values that object’s attributes have at a given time • Can vary, although its OID remains the same • To change the object’s state, change the values of the object’s attributes • To change the object’s attribute values, send a message to the object – Message will invoke a method 7

Messages and Methods • Method: – Code that performs a specific operation on object’s data – Protects data from direct and unauthorized access by other objects – Used to change the object’s attribute values or to return the value of selected object attributes – Represent real-world actions 8

Classes • Collection of similar objects with shared structure (attributes) and behavior (methods) • Class instance or object instance – Each object in a class 9

Protocol • An object’s public aspect • How it is known by other objects as well as end users • Other objects communicate with the student object using any of these methods 10

Object Characteristics 11

Class Hierarchy • Superclass • Subclass • Class lattice 12

Inheritance • Ability of object to inherit the data structure and behavior of classes above it • Single inheritance – Class has one immediate superclass 13

Inheritance (Cont’d. ) • Multiple – Class has more than one immediate superclass 14

Method Overriding • Method redefined at subclass level 15

Polymorphism • Allows different objects to respond to same message in different ways 16

Abstract Data Types (ADT) • Describes a set of similar objects • Differs from conventional data types – Operations are user-defined – Uses encapsulation • Definitions needed for creation – Name – Data representation – Abstract data type operations and constraints 17

Object Classification • Simple – Only single-valued attributes – No attributes refer to other object • Composite – At least one multivalued attribute – No attributes refer to other object • Compound – At least one attribute that references other object • Hybrid – Repeating group of attributes – At least one refers to other object • Associative object 18

OO vs. E-R Model Components 19

Class-Subclass Relationship 20

Interobject Relationships • Attribute-Class Link • Object’s attribute references another object • Relationship Representation – Related classes enclosed in boxes – Double line on right side indicates mandatory – Connectivity indicated by labeling each box • 1: M • M: N with an Intersection Class 21

1: 1 and 1: M Relationships 22

Employee-Dependent Relationship 23

Representing the M: N Relationship 24

Representing the M: N Relationship with Associated Attributes 25

Representing the M: N Relationship with Intersection Class 26

Late and Early Binding • Late binding – Data type of attribute not known until runtime – Allows different instances of same class to contain different data types for same attribute • Early binding – Allows database to check data type at compilation or definition time 27

OODM vs. E-R Data Models • Object, Entity, and Tuple – OODM object has behavior, inheritance, and encapsulation – OO modeling more natural • Class, Entity Set, and Table – Class allows description of data and behavior – Class allows abstract data types • Encapsulation and Inheritance – Object inherits properties of superclasses – Encapsulation hides data representation and method 28

OODM vs. E-R Data Models (Cont’d) • Object ID – Not supported in relational models • Relationships – OODM • Interclass references • Class hierarchy inheritance • Access – Relational models • SQL – OODM • Navigational • Set-oriented access – Relational models • Value-based approach 29

Example Assume the following business rules: • A course contains many sections, but each section has only one course • A section is taught by one professor, but each professor may teach one or more different sections of one or more course • A section may contain many students, and each student is enrolled in many sections, but each section belongs to a different course. (Students my take many courses, but they cannot take many sections of the same course. ) • Each section is taught in one room, but each room may be used to teach several different sections of one or more courses • A professor advises many students, but a student has only one advisor 30

Example (Cont’d) • Identify and describe the main classes of objects • Modify your description in part 1 to include the use of abstract types such as Name, DOB, and Address • Create the conceptual OO representations 31

More Example Using intersection class to represent the following relationship 32

OO Design Example Design OO conceptual representations for an engineering company, using the following requirements: • A customer has a unique customer identifier. Other important attributes of each customer include name and address. They can request any number of work orders from the company. • The company maintains a list of materials. The data about materials include a unique material identifier, a name and cost. • A work order has a unique work order number, a creation date, a completion date, a work address and a set of (one or more) tasks. In addition, each work order has one optional supervising employee. Each employee has a unique number assigned by the company. Other important attributes of each employee include name and skill. • Each work order also has a collection of materials. The same material can be used by any number of work orders. Material requirement includes material quantity. • Each task has a unique task identifier, a task name, an hourly rate and estimated hours. Tasks are standardized across work orders so that the same task may be performed on many work orders. We have to keep record of actual hours of each task on a work order. 33

OO Influences on Relational Model • Extensibility of new user-defined (abstract) data types • Complex objects • Inheritance • Procedure calls (rules or triggers) • System-generated identifiers (OID surrogates) 34