Kendall & Kendall Systems Analysis and Design, 9e

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Systems Analysis and Design, 9e

Entity Relationship Diagrams ERDs

Kendall & Kendall Copyright © 2014 Pearson Education, Inc. Publishing as Prentice Hall 2-2

Entity-Relationship Model• Focus is on the Entities and their

relationships within the organizational system

• Another way to show the scope of a system

• Another way to understand the data of the system

• Will help in database creation

• Will affect programming


Entity-Relationship Model• Entities and their relationships are

important because they have a direct connection with:

• Relational Databases

• See the common word “relation”

• Entity-Relationship Diagram

• Relational Database


Entity-Relationship Model• Relational Databases

• A relational database usually contains many tables

• These tables connect to each other according to their relationship

• Essentially, each entity will quite probablybe a table in a relational database

• Relationships show how the entities are connected


Relationships• For each relationship, you determine which

is the PRIMARY entity

• You will draw the primary entity on the top or on the left

treats

Is treated by Is/are part ofcontains


Relationships• Relationship lines are labeled

• read clockwise

• Use complementary verbs that describe the same relation

treats

Is treated by

contains

Do not draw these arrows in an exercise


Relationships

• Three types of relationships:

• One-to-one

• One-to-many

• Many-to-many

• This is called CARDINALITY

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Relationship Notation

• Cardinality is drawn using

Crow’s Foot Notation

• One:

• If it is only one, draw two short lines

• If the one is combined with another cardinality, just use one line

• Many is indicated by a crow’s foot

• “pata de gallina”


Crow’s Foot Notation


2-10

Figure 2.8Examples of different types of relationships in E-R diagrams


Copyright © 2009 by Prentice Hall, Inc.2-11

One to One

Employee

Phone Extension

IsAssignedTo

IsListedFor

Primary Entity



Many to One / One to Many

Employee

Department

BelongsTo

Is composed

of

Order

Product

ContainsIs composed

of


One to Zero, One or Many

Supplier

Item

Supplies

Are Supplied

By



Many to Many

Flight

Passengers

HaveTravel on


Entity-Relationship Example

An entity-

relationship

diagram

showing a

many-to-one

relationship


Examples of Different Types of Relationships in E-R Diagrams (Figure 2.8)


Attributes

• Data attributes may be added to the diagram.

Patron

Patron NamePatron addressPatron phonePatron credit card

Primary Key - will be your first field, it will be underlined- field that can uniquely identify a record

Is Patron Namea good Primary key?


Attributes


Creating Entity-Relationship Diagrams

• List the entities in the organization

• Choose key entities to narrow the scope of the problem

• Identify what the primary entity should be

• Confirm the results of the above through data gathering

Documents

Kendall & Kendall Systems Analysis and Design, 9e