Data Modeling and the Entity-Relationship Model Chapter Four DAVID M. KROENKE’S DATABASE CONCEPTS, 2 nd Edition

Data Modeling and theEntity-Relationship Model

Chapter Four

DAVID M. KROENKE’S

DATABASE CONCEPTS, 2nd Edition

DAVID M. KROENKE’S DATABASE CONCEPTS, 2nd Edition © 2005 Pearson Prentice Hall

4-2

Chapter Objectives

• Learn the basic stages of database development

• Understand the purpose and role of a data model

• Know the principal components of the E-R data model

• Understand how to interpret both traditional and UML-style E-R diagrams

• Learn to construct traditional E-R diagrams• Know how to represent 1:1, 1:N, N:M, and

binary relationships with the E-R model


4-3

Chapter Objectives (continued)

• Know how to represent recursive relationships with the E-R model

• Understand two types of weak entities and know how to use them

• Learn how to create an E-R diagram from source documents


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Three Stages of Database Development

• Requirements Stage

• Design Stage

• Implementation Stage


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The Requirements Stage

• Sources of requirements– User Interviews– Forms– Reports– Queries– Use Cases– Business Rules


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Requirements Become theE-R Data Model

• After the requirements have been gathered, they are transformed into an Entity Relationship (E-R) Data Model

• E-R Models consist of– Entities– Attributes– Identifiers– Relationships


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Entities

• An entity is something that users want to track– CUSTOMER– PROJECT– EMPLOYEE– STUDENT


4-8

Instance versus Classes

• An entity class is a description of the structure and format of the occurrences of the entity

• An entity instance of a specific occurrence of an entity class


4-9

Instance versus Classes

CUSTOMERCustID

CustName

78124Jackson Co.

12735Smither, Inc

Two EntityInstances

EntityClass


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Attributes

• Entities have attributes that describe the entity’s characteristics– ProjectName– StartDate– ProjectType– ProjectDescription

• Attributes have a data type and properties


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Identifiers

• Entity instances have identifiers

• An identifier will identify a particular instance in the entity class– SocialSecurityNumber– StudentID– EmployeeID


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Identifier Types

• Uniqueness– Identifiers may be unique or nonunique– If the identifier is unique, the data value for the

identifier must be unique for all instances

• Composite– A composite identifier consists of 2 or more

attributes• E.g., OrderNumber & LineItemNumber are both

required


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Relationships

• Entities can be associated with one another in relationships

• Relationship degree defines the number of entity classes participating in the relationship– Degree 2 is a binary relationship– Degree 3 is a ternary relationship


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Degree 2 Relationship - Binary

LOCKER EMPLOYEE


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Degree 3 Relationship - Ternary

DEALER MODEL

MAKE


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One-to-OneBinary Relationship

• 1:1 (one-to-one)– A single entity instance in one entity

class is related to a single entity instance in another entity class


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One-to-OneBinary Relationship

• An employee may have no more than one locker; and

• A locker may only be accessible by one employee

LOCKER EMPLOYEE1:1


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One-to-ManyBinary Relationship

• An employee may only work for one department; and

• A department has several employees

DEPARTMENT EMPLOYEE1:N


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Many-to-ManyBinary Relationship

• An employee may have several skills; and• A particular skill may be held by several

employees

SKILL EMPLOYEEN:M


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One-to-ManyUnary Relationship

• An employee may be managed by one other employee

• A employee may manage several employees

EMPLOYEE 1:N


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Cardinality

• Each of the three types of binary relationships shown above have different maximum cardinalities

• Minimum cardinalities also exist. These values typically assume a value of Mandatory (one) or Optional (zero)


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One-to-One Binary Relationship with Minimum Cardinality

• An employee must have one locker; and • A locker may be accessible by one or zero

employees

LOCKER EMPLOYEE1:1 0|


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Weak Entity

• A weak entity is an entity that cannot exist in the database without the existence on another entity

• For example, an employee’s dependents cannot exist in a database without the employee existing in the database

EMPLOYEE 1:N 0| DEPENDENT


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ID-Dependent Weak Entities

• An ID-Dependent weak entity is a weak entity that must include the identifier of its parent entity as part of its composite primary key

BUILDING 1:N 0| APARTMENT


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Weak Entity Identifier:Non-ID-dependent

• A non-ID-dependent weak entity may have a single or composite identifier, and the identifier of the parent entity will be a foreign key

PATIENT 1:N 0| PERSCRIPTION


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Weak Entity Identifier:ID-Dependent

• An ID-dependent weak entity has a composite identifier– The first part of the identifier is the

identifier for the strong entity– The second part of the identifier is the

identifier for the weak entity itself

PROJECT 1:N 0| ASSIGNMENT


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Weak Entity Relationships

• The relationship between a strong and weak entity is termed an identifying relationship if the weak entity is ID-dependent

• The relationship between a strong and weak entity is termed a non-identifying relationship if the weak entity is non-ID-dependent


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Unified Modeling Language Entity-Relationship

Diagrams• Unified Modeling Language (UML) is

a set of structures and techniques for modeling and designing object-oriented programs (OOP) and applications


4-29

Unified Modeling Language Entities

ENTITY_NAME

List of Attributes

Identifier


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Unified Modeling Language Relationships

0..1

0..*

1..*

1..1 Mandatory One

Optional One

Optional Many

Mandatory Many


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UML E-R Diagram Example

• An employee must report to one department; and

• A department may have zero or many employees

EMPLOYEE

EmployIDEmployNamePhone

EmployID is identifier

DEPARTMENT

DeptIDDeptNameLocation

DeptID is identifier

0..* 1..1


4-32

UML Weak Entity

EMPLOYEE

EmployIDEmployNamePhone

EmployID is identifier

DEPENDENT

DepSSNDepNameDepAge

DepSSN is identifier

10..N

Data Modeling and theEntity-Relationship Model

End of Presentation on Chapter Four

DAVID M. KROENKE’S

DATABASE CONCEPTS, 2nd Edition

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Data Modeling and the Entity-Relationship Model Chapter Four DAVID M. KROENKE’S DATABASE CONCEPTS, 2 nd Edition