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Chapter 7

Chapter 7

Data Modeling Using the Entity-

Relationship (ER) Model

Entity Relationship Model (ER)

Entity Relationship Model (ER)



ER model was proposed by

ER model was proposed by Peter Chen

Peter Chen in

in



ER model has become the standard tool for

ER model has become the standard tool for

conceptual schema

conceptual schema design

design



ER model consists of three basic constructs:

ER model consists of three basic constructs:

entities

entities ,

attributes

attributes and

and relationships

relationships .

Entity and Entity Set

Entity and Entity Set



Two types of entities:

Two types of entities:



Strong entity: can exist independently (or

can uniquely identify itself)



Weak entity: existence depends on the

existence of other (strong) entity or

entities



Examples:

Examples:



An employee is a strong entity but the

dependents of the employee could be

weak entities



An account in a bank is a strong entity but

a transaction could be a week entity

Entity and Entity Set

Entity and Entity Set



An

An

entity type

entity type

defines a set of entities that

defines a set of entities that

have the same attributes.

have the same attributes.



STUDENT is an entity type (Schema)



An

An entity set

entity set is a collection of entities of the

is a collection of entities of the

same entity type

same entity type



Examples:

Examples:



Rema, Ali, Amal, Samer, Rana are entity

set of an entity type STUDENT

Types of Attributes

Types of Attributes



Simple

Simple (or

(or atomic

atomic ) attribute is a one which

) attribute is a one which

cannot be divided into smaller parts.

cannot be divided into smaller parts.



Examples:



SSN, GPA, Salary.



Composite

Composite attribute is an attribute which can

attribute is an attribute which can

be divided into smaller subparts, these subparts

be divided into smaller subparts, these subparts

represent more basic attributes with

represent more basic attributes with

independent meanings of their own

independent meanings of their own



Examples:



Name: First_Name, Middle_Name, Last_Name



Address: Street_Address, City, State, Zip code

House No.

Address

Street-Address City

State Post Code

Street Name

An Example of a composite attribute

An Example of a composite attribute

Types of Attributes

Types of Attributes



A

A

stored

stored attribute is a one whose value is

attribute is a one whose value is

explicitly stored in the database.

explicitly stored in the database.



e.g. name, birth-date.



Derived

Derived -attributes: whose values are

-attributes: whose values are

computed from other attributes.

computed from other attributes.



Age from Birthdate



Annual Salary from Monthly Salary



NoOfEmployees ==> Count number of

employees in the Employee table.

Relationship Types

Relationship Types



A relationship type is represented as

A relationship type is represented as diamond-

diamond-

shaped box

shaped box

which is connected by straight lines

which is connected by straight lines

SUPPLIER

PROJECT

PART

supplies

Ternary Relationships

Ternary Relationships

COMPANY

PRODUCT

COUNTRY

Sells

Ternary Relationships

Relationships

Relationships



The

The role name

role name signifies the role that a

signifies the role that a

participating entity from the entity type

participating entity from the entity type

plays in each relationship instance

plays in each relationship instance



entity

entity e

e plays the role of a

plays the role of a supervisor

supervisor , while

, while

entity

entity e

e plays the role of a

plays the role of a supervisee

supervisee

EMPLOYEE

Supervises

supervisor

supervisor

supervisee

supervisee

e

e

e

e

Cardinality Ratio

Cardinality Ratio



Specifies the

Specifies the number

number of

of relationship instances

relationship instances

that an entity can participate in

that an entity can participate in



Common cardinality ratios for

Common cardinality ratios for binary

binary

relationship types are

relationship types are





1:N , and



M:N

DEPARTMENT Has

MANAGER

A department has one manager and a

manager manages one department

EMPLOYEE

Works-on

PROJECT

M N

An employee works on many projects,

and a project has many employees

working on it

M:N

M:N