Conceptual DesignConceptual DesignRevisionRevision

Database DevelopmentDatabase Development

Basic constructs of theBasic constructs of theEntity Relationship ModelEntity Relationship Model

EntityEntity Representation of an Entity on an ER Representation of an Entity on an ER

diagram is:diagram is:– a box with square corners a box with square corners – labelled with the entity name.labelled with the entity name.

Note – the entity name is always singularNote – the entity name is always singular

Student

Basic constructs of theBasic constructs of theEntity Relationship ModelEntity Relationship Model

Weak EntityWeak Entity Representation of a Weak Entity on Representation of a Weak Entity on

an ER diagram is:an ER diagram is:– a box with rounded corners a box with rounded corners – labelled with the entity name (singular)labelled with the entity name (singular)

NextOfKin

Basic constructs of theBasic constructs of theEntity Relationship ModelEntity Relationship Model

Strong EntityStrong Entity By default an entity which is not a By default an entity which is not a

weak entity is a strong entity.weak entity is a strong entity.

Student

ER Model Relationship ER Model Relationship TypesTypes

One to OneOne to One– An employee is allocated a single An employee is allocated a single

parking space, a parking space belongs parking space, a parking space belongs to only one employee.to only one employee.

1:1Employee ParkingSpace

ER Model Relationship ER Model Relationship TypesTypes

One to ManyOne to Many– A department has many employees A department has many employees

assigned to it, each employee is assigned to it, each employee is assigned to only one department.assigned to only one department.

1:MDepartment Employee

ER Model Relationship ER Model Relationship TypesTypes

Many to ManyMany to Many– An employee attends many staff An employee attends many staff

development sessions, each staff development sessions, each staff development session is attended by development session is attended by many employees.many employees.

N:MStaffDevSess Employee

ER Model Relationship ER Model Relationship TypesTypes

Many to ManyMany to Many– An employee attends many staff An employee attends many staff

development sessions, each staff development sessions, each staff development session is attended by development session is attended by many employees.many employees.

N:MStaffDevSess Employee

ER Model Relationship ER Model Relationship TypesTypes

InheritanceInheritance– A technician is a A technician is a

kind of employee kind of employee about whom we about whom we wish to record, in wish to record, in addition to his addition to his other attributes, his other attributes, his highest technical highest technical qualification.qualification.

Technician

Employee

ER Model Relationship ER Model Relationship ExistenceExistence

In the ER model optional association In the ER model optional association existence is shown as.existence is shown as.

An employee may be a member of An employee may be a member of one of the works football teams.one of the works football teams.

1:MFootballTeam Employee

ER Model Relationship ER Model Relationship ExistenceExistence

In the ER model mandatory In the ER model mandatory association existence is shown as.association existence is shown as.

An employee must be a member of An employee must be a member of one of the unions recognised by the one of the unions recognised by the company.company.

1:MUnion Employee

ER Model RelationshipER Model Relationship

Recursive relationship.Recursive relationship. An employee may also be a An employee may also be a

manager. A number of employees manager. A number of employees are managed by a single managerare managed by a single manager

Employee

1:MEmployee Manager

AttributesAttributes

What is an attribute?What is an attribute? An attribute is a piece of information An attribute is a piece of information

about the object or event that we wish about the object or event that we wish to store in our database.to store in our database.

It can be said to describe the object.It can be said to describe the object. An example of some possible attributes An example of some possible attributes

for Employee could be EmployeeID, for Employee could be EmployeeID, EmpFirstName, EmpSurnameEmpFirstName, EmpSurname

ER Model Representation of ER Model Representation of AttributesAttributes

Only attributes which uniquely Only attributes which uniquely identify an entity will be represented identify an entity will be represented on your ER diagram.on your ER diagram.

A key attribute may be represented A key attribute may be represented as followsas follows

Employee

EmployeeID

ER DiagramER Diagram

STUDENT HALLN:1

StudentID HallName

Entity Entity Name

Relationship

Cardinality

Attribute Attribute name

Mandatory Existence

Optional Existence

Diagram ScriptDiagram Script

Entity name Attributes Identified Synonym Related to

Course titlecourseCode

Prerequisite StudentClassCourse

Student studentIDstudentNameaddresspostcode

Course

Class semesterclassRoomuniqueClassNametutor

Course

Relationship between course and student

grade Course and Student

Using the Information BaseUsing the Information Base

Where do we start?Where do we start? Analyse the information gathered Analyse the information gathered

during the requirements analysis.during the requirements analysis.– One element at a timeOne element at a time

Identifying EntitiesIdentifying Entities

A noun is a word that names a person or A noun is a word that names a person or thing thing

A noun phrase is a group of words that A noun phrase is a group of words that operate like a noun e.g. dentist operate like a noun e.g. dentist appointmentappointment

Consider each noun or noun phrase and Consider each noun or noun phrase and determine if it is a candidate entity.determine if it is a candidate entity.

Reasons for rejecting candidate entities.Reasons for rejecting candidate entities.

Rejecting Candidate EntitiesRejecting Candidate Entities

RedundancyRedundancy VaguenessVagueness An event or operationAn event or operation Meta LanguageMeta Language Outside the system boundaryOutside the system boundary AttributeAttribute

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In-class testIn-class test