Datamodelling

Systems Analysis and DesignSystems Analysis and Design
5th Edition5th Edition
Chapter 6. Data ModelingChapter 6. Data Modeling
Alan Dennis, Barbara Haley Wixom, and Roberta Roth
6-1© Copyright 2011 John Wiley & Sons, Inc.

Chapter 6 Outline
 The Entity Relationship Diagram
(ERD).
- Elements of ERD
- The Data Dictionary and Metadata
 Creating an Entity Relationship
Diagram.
 Validating an ERD.
© Copyright 2011 John Wiley & Sons, Inc. 6-2

INTRODUCTION
 In this chapter, we discuss how the data are
organized and presented.
 A data model is a formal way of representing the
data that are used and created by a business system.
 During analysis (this Chapter), analysts draw a
Logical data model which shows the logical
organization of data without indicating how it is
stored, created, or manipulated.
 During design (Chapter 11), analysts draw a physical
data model to reflect how the data will physically be
stored in databases or files.

(cont’d)
 Topics of this chapter:
- Creating an entity relationship diagram
(ERD).
- Normalization, a technique that helps
analysts validate the data models.
- How data models balance, or
interrelate, with the process models.

THE ENTITY-RELATIONSHIP DIAGRAM
(ERD)
An entity-relationship diagram (ERD) is a
picture showing the information that is
created, stored, and used by a business
system.
Entities lists similar kinds of information
Lines drawn between entities represent
relationships among the data.
Special symbols communicate high-level
business rules.

Reading an Entity Relationship
Diagram

Elements of an Entity Relationship
Diagram

Entity
The entity is the basic building block for
a data model. It is a person, place,
event, or thing about which data is
collected.
 Entities represent something for which
there exist multiple instances, or
occurrences.
- E.g., John Smith could be an instance of the
customer entity.

(cont’d)

Attributes
An attribute is some type of information that
is captured about an entity.
Attributes are nouns that are listed with an
entity.
One or more attributes can serve as the entity
identifier - the attribute(s) that can uniquely
identify one instance of an entity.
Concatenated identifier - several attributes
are combined to uniquely identify an instance.

(cont’d)
Choices for Identifiers

Relationships
Relationships are associations
between entities.
Every relationship has a parent
entity and a child entity.
Relationships should be labeled with
active verbs.

Cardinality
 A relationship has cardinality which is the ratio of
parent instances to child instances.
 The 1:1 relationship means that one instance of the
parent entity is associated with one instance of the
child entity.
 The 1:N relationship means that a single instance of
a parent entity is associated with many instances of a
child entity.
 The M:N relationship means that many instances of
a parent entity can relate to many instances of a
child entity.

(cont’d)
Example of M:N Relationship

Modality
A relationship has modality of null or not null,
which refers to whether or not an instance of a
child entity can exist without a related instance in
the parent entity.
Null means that an instance of a child entity can
exist without a related instance in the parent
entity.
Not Null means that an instance of a child entity
can’t exist without a related instance in the
parent entity.

The Data Dictionary and Metadata
A data dictionary contains the information
about the entities, attributes, and
relationships on the ERD, or metadata.
Metadata is data about data.
Metadata is stored in the data dictionary so
it can be shared by developers and users
throughout the SDLC.

(cont’d)
Example of Data Dictionary Entry for
Entity

(cont’d)
Example of Data Dictionary Entry for
Attributes

(cont’d)
Example of Data Dictionary of Entry
for Relationship

(cont’d)
Types of Metadata Captured by the
Data Dictionary

CREATING AN ENTITY RELATIONSHIP
DIAGRAM (ERD)
Drawing the ERD is an iterative
process of trial and revision.
The basic steps in building an ERD:
1. Identify the entities;
2. add the appropriate attributes to
each entity; and
3. draw relationships among entities.

Step 1: Identify the Entities
 The entities should represent the major
categories of information that you need
to store in your system.
If you begin the data model using a use
case, look at the major inputs and
outputs of the use case.
If the process models are available, look
at the data stores, external entities, and
data flows.

Step 2: Add Attributes and Assign
Identifiers
The information that describes each entity
becomes its attributes.
– Check in the CASE repository of the process model for
details on data flows and data stores.
– Check the data requirements of the requirements
definition.
– Use requirements elicitation techniques (e.g., interview
and document analysis).
One or more of the attributes will become the
entity’s identifier.

Step3: Identify Relationships
The last step in creating ERDs is to
determine how the entities are
related to each other.
Lines are drawn between the entities
that have relationships.
Each relationship is labeled, and
cardinality and modality are
assigned.

Advanced Syntax
Three special types of entities:
Independent Entity
–Can exist without the help of another
entity.
–The identifiers is created from the entity’s
own attributes.
–Attributes from other entities are not
needed to uniquely identify instances of
these entities.

(cont’d)
Dependent Entity
– There are situations when a child entity does
require attributes from the parent entity to
uniquely identify an instance. In these cases,
the child entity is called a dependent entity, and
its identifier consists of at least one attributes
from the parent entity.
(E.g., the Chemical Request entity in Figure 6.1).

(cont’d)
Intersection Entity
–It exists in order to capture some
information about the relationship
between two other entities. Typically,
intersection entities are added to a
data model to store information about
two entities sharing an M : N
relationship.

(cont’d)
There are three steps involved in adding an
intersection entity:
Step 1. Remove the M:N relationship line and
insert a new entity (intersection entity) in
between the two existing ones.
Step 2. Add two 1:N relationships to the
model.
Step 3. Name the intersection entity.

(cont’d)
Resolving an M:N Relationship

VALIDATING AN ERD
 General design guidelines.
 Normalization.
Check the ERD against the process
models to make sure that both
model balance each other.

Design Guidelines

Normalization
Normalization is a technique that
can help analysts validate the data
models.
It is a process whereby a series of
rules are applied to a logical data
model to determine how well
formed it is.

(cont’d)
Normalization Steps

Balancing ERDs with DFDs
 The process models and data models are
interrelated.
 Although the process model focuses on the business
processes, it contains two data components – the
data and the data store.
 These two data components of the DFDs need to
balance with the ERDs.
 The DFD data components need to correspond with
the ERD’s data stores (i.e., entities) and the data
elements that comprise the data flows (i.e.,
attributes) depicted on the data model.

(cont’d)
 Many CASE tools provide features of identifying
problems with balance between DFDs and ERDs;
however, it is important to understand how to
identify problems on your own.
 Check your DFDs and ERDs to make sure all data
components correspond between DFDs and ERDs.
 A useful tools to clearly depict the interrelationship
between process and data models is the CRUD
matrix (create, read, update, delete matrix).

(cont’d)
A Portion of a DFD and the CRUD Matrix

SUMMARY
 Basic Entity Relationship Diagram Syntax
- Entity describes people, places, or things.
- Attribute is some type of information about the entity.
- Relationship conveys the associations between entities.
 Creating an ERD
- Identify the entities.
- Add the attributes to each entity.
- Draw relationships among entities.
 Validating an ERD
- Normalization
- CRUD matrix

Copyright 2011 John Wiley & Sons, Inc.
All rights reserved. Reproduction or translation of this work
beyond that permitted in Section 117 of the 1976 United States
Copyright Act without the express written permission of the
copyright owner is unlawful. Request for further information
should be addressed to the Permissions Department, John Wiley
& Sons, Inc. The purchaser may make back-up copies for his/her
own use only and not for redistribution or resale. The Publisher
assumes no responsibility for errors, omissions, or damages,
caused by the use of these programs or from the use of the
information contained herein.

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