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1

Objectives:

To illustrate how relationships between entities are defined and

refined.

To know how relationships are incorporated into the database

design process.

To describe how ERD components affect database design and

implementation.

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2

Entity Relationship Diagram (ERD)

A detailed, logical representation of the entities,associations and data elements for anorganization or business

Notation uses three main constructs :

Entities

Relationships Attributes

Chen Model or

Crows Foot

Model

DEFINITION

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3

DIFFERENCES OF MODEL

Crows Foot Chen

Entity

Attribute

Relationship

Department

Staff No.

employs

Staff No

Department

Department2

2

employs

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Person, place, object, event

or concept about which data

is to be maintained

named property or

characteristic of an

entity

Association

between the

instances of one or

more entity types

Represents a set or collection of

objects in the real world that

share the same properties

Chen Notation

EntityName Verb Phrase AttributeName

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ENTITY

An entity is a thing of interest to a system about whichinformation is kept

For example in a Hospital Administration System, some likelyentities would include: Patient, Doctor, Operation, Ward

Each of these things are of interest to the system and wouldhave data stored about them

5

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ENTITIES EXAMPLES

Examples of entities: Person: EMPLOYEE, STUDENT, PATIENT

Place: STORE, WAREHOUSE

Object: MACHINE, PRODUCT, CAR

Event: SALE,REGISTRATION, RENEWAL

Concept: ACCOUNT, COURSE

Guidelines for naming and defining entity types: An entity type name is a singular noun

An entity type should be descriptive and specific An entity name should be concise

Event entity types should be named for the result of the event,not the activity or process of the event.

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ATTRIBUTE

An attribute is an item of data held about an entity

In computer systems an attribute is a field of information

Example : Sales System

Entity : Customer

Attributes : Customer Name, Customer Address, Customer Phone

Number

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Entity : STUDENT

Attributes: Student_ID, Student_Name, Home_Address,Phone_Number, Major

Guidelines for naming attributes:

An attribute name is a noun.

An attribute name should be unique

To make an attribute name unique and clear, each attribute

name should follow a standard format

Similar attributes of different entity types should use similarbut distinguishing names.

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KEYS

A key attribute uniquely identifies a specific occurrence of an entity

This may also be referred to as a primary key

Examples of primary keys include:

ISBN

Student Number

Staff ID

10

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EXAMPLE

In a Sales System, customer name and address are

attributesof the entitycustomer.

The primary keyis likely to be Customer Code or

Customer ID as the unique identifier for each customer in the

system.

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EXAMPLE : BABYSITTERSERVICE

The AITP Service Club wants to run a babysitting service.

Customers call to request a sitter and the Club Coordinator

assigns an employee to sit for the customer from a list of

employees available for the particular day requested.

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BABYSITTING SYSTEM

Data is used to:

Assign employee to job

Determine availability

Data to be captured in database:

Employee data

Job data

Customer data

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BABYSITTING SYSTEM

Employee

Attributes: EmpID, Name, Address,

Phone Number, Available Hours,JobNo Job

Attributes: JobNo, JobDate, JobTime,EmpID

Customer

Attributes: CusNo, Name, Address,Phone Number, JobNo

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ERD - Babysitter Service

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ERD - Babysitter Service

EMPLOYEE

JOB

CUSTOMER

Related toassigned to

1

MN

M

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TO BE CONTINUED

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FOREIGN KEYDefinition

A foreign key is a relationship or link between two tableswhich ensures

that the data stored in a database is consistent.

Example:

In the example, there is a link between the Company and Contact tables.

The Company table is the parent table in the link. The Contact table is the

child: the Company_ID field in the Contact table indicates which Company

a Contact belongs to.

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RELATIONSHIPS

DEFINITION:

Associations between instances of one or more entity types

that is of interest

Author Book

Relationship name:write

An author writes one or more books

A book can be written by one or more authors.

write

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DEGREE O F RELA T IO NSHIPS

Degree: number of entity types that participate in a relationship

Three cases

Unary: between two instances of one entity type

Binary: between the instances of two entity types

Ternary: among the instances of three entity types

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C ARD INAL ITY AND

CONNECTIVITY

Relationships can be classified as either one to one ( 1:1 )

one to many (1:M)

many tomany (M:N)

Cardinality: minimum and maximum number

of instances of Entity B that can (or must be)

associated with each instance of Entity A.

Connectivity

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2 . 4 C A R D IN A L ITY A N D C O N N E C TIVITY

A professor teaches class OR

A class is taught by professor

How Many??

PROFESSOR CLASSteaches

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CARDINALITY AND CONNECTIVITY

1 MPROFESSOR CLASSteaches

Connectivity

(1,1)(1,4)

Cardinality

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CONNECTIVITY

Chen Model

1 to represent one

M to represent many

1

M

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BINARY RELATIONSHIPS

1:M relationship

Relational modeling ideal

Should be the norm in any relational database design

The 1: M relationship between PAINTER and PAINTING

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BINARY RELATIONSHIPS

1:1 relationship

Should be rare in any relational database design

A single entity instance in one entity class is related

to a single entity instance in another entity class

Could indicate that two entities actually belong in

the same table

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BINARY RELATIONSHIPS

M:N relationships

Can be implemented by breaking it up to produce a set of

1:M relationships

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The M:N Relationship Between STUDENT and CLASS

This CANNOT be implemented as shown next..

Bowser

Smithson

Accounting 1 (ACCT-211)

Intro to Microcomputing (CIS-220)

Intro to Statistics (QM-261)

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~The END~

Q & A