1 Version 1.0 02/05/2004 © 2004 Robert Oshana Requirements Engineering Entity relationship diagrams.

Version 1.0 02/05/2004© 2004 Robert Oshana

Requirements Engineering 1

Entity relationship diagrams



What is an ERD?

• Abstractions of the real world which simplify the problem to be solved while retaining its essential features

• Used to;– Identify the data that must be captured, stored,

and retrieved in order to support the business activities performed by the organization

– Identify the data required to derive and report on the performance measures that an organization should be monitoring



What is an ERD?

• Must be clearly defined so that all understand exactly what is being represented

• Relationships are evaluated in both directions to determine what type of relationship exists– One friend may have many telephones– One telephone belongs to a single friend



Components of ERD

• Entities• Attributes• Relationships



What’s in an ERD?

• Entities are drawn in boxes• Entities should be expressed in plural• Relationship is a line connecting the

entities• Arrows represent different types of

relationships• List of attributes developed as you go• Verbs placed on relationship lines that

describe relationship



Entities

• People, places, things, events, concepts of interest to an organization

• Anything the organization needs to store data about

• Represented by labeled boxes• Collections of things



Entities

Aircraft Wedding

Customer Sale



Entities

• EMPLOYEE; collection of employees that work at an organization

• Individual members (employees) of the collection are called occurrences of the EMPLOYEE entity

• Entities should have detailed descriptions (space limited inside the box)



Entities

• Further described by attributes or data elements

• Smallest units of data that can be described in a meaningful manner

Employee

Employee #SurnameGiven nameDate of birthTelephone #Department



Relationship

• Frequently, a meaningful relationship exists between two different types of entity– EMPLOYEE works in a DEPARTMENT– LAWYER advises CLIENTS– EQUIPMENT is allocated to PROJECTS– TRUCK is a type of VEHICLE



Types of Relationships

• One-to-One relationships• One-to-Many relationships• Many-to-Many relationships



One-to-One relationships

• Takes place when a single occurrence of an entity is related to just one occurrence of a second entity– A ROOF covers one BUILDING– A BUILDING is covered by one ROOF



One-to-One relationships

Roof

Building

Covered by

Covers



One-to-Many relationships

• Takes place when a single occurrence of an entity is related to many occurrences of a second entity– EMPLOYEE works in one DEPARTMENT– A DEPARTMENT has many EMPLOYEES



One-to-Many relationships

Department

Employee

Works in

has



Many-to-Many relationships

• Takes place when many occurrences of an entity are related to many occurrences of a second entity– EQUIPMENT is allocated to many PROJECTS– PROJECT is related to many items of

EQUIPMENT



Many-to-Many relationships

Equipment

Project

allocated



Choosing the right relationship

• Type of relationship can change– Purpose of the model– Length of time involved– Definition of the entities participating in the

relationship

• If the definition of a ROOF entity is an apex or flat surface covering a building, then a BUILDING is covered by many ROOFS



Choosing the right relationship

• Likewise, over time an EMPLOYEE works in many DEPARTMENTS



Tips on building ERDs

• Nouns are clues to the entities of a business

• Start by describing the work that is done in the project or department or area of interest– Pull put nouns in the sentences as potential

candidates for entities




• “My address book contains addresses and telephone numbers for both friends and businesses”




• Verbs and adjectives sometimes help to get at relationships between entities– Friends have addresses

• Many-to-many relationships will cause you lots of trouble!– Get rid of them by defining an intermediate third

entity—Friends/kids between “Kids” and “Friends”




• Spend the time to accurately define your entities so that everyone clearly understands what they represent– You will save yourself a lot of time and

headaches later in the process

• A step by step worksheet can also be helpful

1 Version 1.0 02/05/2004 © 2004 Robert Oshana Requirements Engineering Entity relationship diagrams.

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