CS 4310: Software Engineering

Lecture 5

Entity Relationship Modelling

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Entity-Relationship Model

Technique for carrying out the conceptual and logical design of the system

A widely accepted data modelling approach

3 basic notions:

» entities» attributes» relationships

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What is an entity?

An entity is an object that can be identified in the users’ work environment and that users want to track.

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• An entity is a thing or object in the real world (within the application context)

• An entity has a set of properties which uniquely identify it.

• An entity is represented as a rectangle in an ER diagram

Entities

Project

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What is an Attribute?

An attribute describes a characteristic of an entity

For example:- An entity: Employee

- Has attributes:

» Employee_Name

» Extension

» Date_Of_Hire

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Example: Project = (proj_name, location, budget,

start_date, end_date)

Represented as ellipses in an ER diagram

Projectproj_name

locationbudget

start_date

end_date

Attributes

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What are Relationships?

Relationships are associations between entities which express some real world relationship

Project EmployeeEmploys

• Project and Employee participate in the employs relationship

• The function that an entity plays in a relationship is called that entity’s role

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Road Town

ends at

starts at• There can be more than

one relationship between entities.

• There can be recursive relationships that can indicate roles for clarity.

• A relationship can also have descriptive attributes.

Employee works for

Client Bookorders

date

manager

worker

Relationship Sets

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Degree of Relationships

Refers to the number of entities participating in a relationship

Most relationships are binary (degree 2), but can also have ternary (degree 3) relationships.

Project ContractorHires

Contract

number

value

start_date

end_date

Name

Address

proj_name

location

budget

start_dateend_date

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Express the number of entities to which another entity can be associated via a relationship

For a binary relationship, the mapping cardinality can be one of the following:

• one to one

• one to many

• many to one

• many to many

Mapping Cardinalities

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Cardinality Examples

A person has only one ID book and an ID book belongs to only 1 person

A person can own many cars (possibly 0), but a car only has 1 owner

A person can own many houses and a house can have multiple owners

Person ID Bookowns

Person Carowns

Person Houseowns

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Design Issues

Entity or Attribute?

• Example: should location be an entity or an attribute?

• can a project be in more than 1 location?

Entity or Relationship?

• generally: relationships for actions between entities

• whatever “makes sense” in application context

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ER Diagram Symbols

Entity Set

Attribute

Relationship

One-to-one link

Many-to-one link

Many-to-many link

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Example ERD

Project Employeeemploys

Alienspecies

area

start_date

first_name

position

proj_name

location

budgetstart_date

end_dateID surname

year_joined

year_leftend_date

eliminates

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ER Design: Where to Start

• Identify entity sets

• Describe the entity sets with attributes

• Specify relationships and cardinalities

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• Every department within the company is in only one division.

• Each division has more than one department in it.

• There is no upper limit on the number of departments that a division can have.

Simple Modelling Example

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Division

Department

Employee

1. Define Entities

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2. Define Relationships

Division

Department

Employee

contains

manages

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Division

Department

Employee

contains

manages

3. Define Cardinality

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Standard Notation

INVOICE

is sent by

A B

A B

A plain rectangle is used to represent the entity type.

A labeled line to represent the relationship.

1 : 1

1 : N

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Diagram with Standard Notation

Division

Department

Employee

contains

manages

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COURSE STUDENThas enrolled

LOAN BOOKrefers to

ONE loan refers to ONE book.ONE book is referred to ONE loan.

ONE course has enrolled ONE or MORE students.ONE student is enrolled on ONE course.

Notation Examples

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• Any one supplier might supply more than one kind of part.

• Any one kind of part might be bought from a number of different suppliers.

SUPPLIER PARTsupplies

Many-to-Many Relationship Example

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More Examples

Customer Order

Customer Property

Teachers Studentsteach

can place

is associated

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WARD

NURSE PATIENT

DOCTOR

treatsaccommodateshas

assigned

AILMENTsuffersfrom

caresfor

Example - Simple Hospital System

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DEPARTMENT

LECTURER

STUDENT

COURSE

is_in

enrols

teaches

offers

College System

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Your Project Work

• Develop Requirements Document.

• Think about your system’s entities and relationships