Object Orientated Database Management Systems

8/3/2019 Object Orientated Database Management Systems

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Object Orientated DatabaseManagement Systems

Team Nutmeg V2 Bock Yang


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What is RDBMS?

Relational Database ManagementSystem(RDBMS)

RDBMS requires tabular structure fordata

RDBMS is designed to take care oflarge amounts of data and secure itunlike DBMS


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Reasons for Unsuitability ofRDBMS Poor Representation of "Real World"

Entities

Normalization leads to relations that does

not correspond to entities in real world. Semantic Overloading

Relational model has only one construct

for representing data and datarelationships: the relation


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Reasons for Unsuitability ofRDBMS Poor support for Integrity and

Enterprise Constraints

Homogeneous Data Structure Relational model assumes both horizontal

and vertical homogeneity

RDBMS has limited operations whichonly have a fixed set of operations thatcannot be extended


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Reasons for Unsuitability ofRDBMS Difficulty handling of Recursive

Queries

Recursive queries are queries about

relationships that a relation has with itself(directly or indirectly)

Extension proposed to relational

algebra to handle this type of query isrecursive closure operation (i.e. tuplesare transitively deduced)


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Recursive Example

SELECT managerstaffNo

FROM Staff

WHERE staffNo=S005

UNION

SELECT managerstaffNo

FROM Staff

WHERE staffNo=

(SELECT managerstaffNo

FROM Staff

WHERE staffNo=S005)


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Reasons for Unsuitability ofRDBMS Other Problems with RDBMS

Transactions are generally short-lived andconcurrency control protocols not suited

for long-lived transactions

Schema changes are difficult

RDBMS are poor at navigational access


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OO Concepts

Abstraction, encapsulation,information hiding.

Objects and attributes.

Object identity. Methods and messages.

Classes, subclasses, superclasses,

and inheritance. Overloading.

Polymorphism and dynamic binding.


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Abstraction

Denotes extracting of essential detailsabout items or a group of items whileignoring the inessential details.

Focus on attributes of an object beforedeciding on the implementation

methods.


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Encapsulation and InformationHiding*Note: Encapsulation is not informationhiding

Encapsulation: TO enclose related

operations and data related to anobject into that object

Information Hiding: Hiding certain

details which will not affect other partsof the system.


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Object

Superkey identifiable entity that haveboth attributes and actions associatedwith it.


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Attributes

Contains current state of an object

Classified into simple or complex

Simple attributes can be primitive type

such as integer, string, etc.

Complex attributes contain collectionsor references


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Object Identity

Object Identifier must be

System-generated

Unique

Invariant

Independent of the values of itsattributes

Invisible to user


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Mapping object into RDBMS

Requires mapping class instances toone or more tuples with distributedrelations

Have to handle 2 basic class hierarchytasks

1. Design relations to represent hierarchy

2. Design how objects will be accessed


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Object Identity -Implementation In RDBMS, object identity is value-

based: primary key is used to provideuniqueness.

Primary keys do not provide type ofobject identity required in OOsystems:

key only unique within a relation, notacross entire system

Key generally chosen from attributes ofrelation, making It dependent on object

state


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Methods and Messages

Method Defines behaviour of an object, as set of

encapsulated functions

Message

Request from one object to another

asking secondary object to execute one ofits methods.


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Object Showing Attributes andMethods


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Example of a Method


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Class

Blueprint for defining a set of similarobjects.

Objects in a class are called

instances. Class is also an object with own class

attributes and class methods.

Class different from type


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Class Instance Share Attributesand Methods


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Mapping Classes to Relations

Example


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(1) Map each class or subclass to arelation:

Lecturer(StaffID, homeNO,ExtensionNO, position, gender)

Course Manager(StaffID, course,

lecturers)Course Coordinator(StaffID, students)

Subject Manager(StaffID, Subject, labs)


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(2) Map each subclass to a relation

Course Manager(StaffID, homeNO,

ExtensionNO, position, gender, course,lecturers)

Course Coordinator(StaffID, homeNO,ExtensionNO, position, gender ,students)

Subject Manager(StaffID, homeNO,ExtensionNO, position, gender, Subject, labs)


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(3) Map the hierarchy to a singlerelation

Lecturer(StaffID, homeNO, ExtensionNO,position, gender ,course, lecturers ,students ,subject, labs)

Object Orientated Database Management Systems

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