Entity Relationship Modelling

Database Development A Relational modelIts E-R equivalentE-R notationDeveloping an E-R ModelE-R difficulties

Database Development

From UoD + requirement To implemented systemHow to structure the activity?Don’t do everything at onceSequence decisions - Users first,

technology lastRelational model too close to

technology

Why not Relational schema directly?

Which DBMS will be used? Oracle - 7, 8, 9, Jet, MySQL?

Maybe another Data model altogether? OLAP, Object oriented, Logical, XML

Relational schema too far from the user - some concepts are not user terms

e.g. primary keys, foreign keys

many tables in schema are artifactse.g. resolving many-many relationships

some relationships not explicit in schema e.g. employee and salgrade, no self-managers

A Relational Schema

Dept (Deptno, Dname, Loc)Emp(Empno, ename, job, mgr,

hiredate, sal, comm, deptno)Salgrade(grade, losal, hisal)

Only tables and columns, but columns used for different purposes - what are they?

Different kinds of column

Primary key - UnderlineForeign Key - ItalicAttribute

Dept (Deptno, Dname, Loc)Emp(Empno, ename, job, mgr,

hiredate, sal, comm, deptno)Salgrade(grade, losal, hisal)

More abstract - not specific to relational data model

Entity - cf- TableRelation - cf Primary Key/Foreign keyAttribute - cf most columns

Sometimes called an EAR model

Entity Relation model

E-R <> Relational Schema

Relational Schema >>> E-R Model We will first use E-R models as a way to

represent a Relational Schema, using the example database from the SQL workbook

Uod >>> E-R model >>> Schema We will then use an E-R model as a step

in translating user requirments into a database schema

E-R model of Employee DBEntity: Department

Attributes: Name, LocationEntity: Employee

Attributes: Name, Job, Hiredate, MonthlySalary, Commission

Entity: SalaryGrade: Attributes: Grade, losal, hisal

Relationship: Manages (Employee, set of Employee)

Relationship: Employs(Department,set of Employees)

E-R Diagrams

E-R models are often represented as diagrams.

Diagrams help to show Relationships clearly

No single notation, so ensure you use a legend to explain your notation

E-R diagrams

Entity

Relationship 1 to a set of (many)

Attribute

Department

name

Employee

E-R instance diagram

E1 E2 E5 E3 E4

D1 D2

Entity Questions

Entities are complex things of interest in the UoD

Entity corresponds to Table in Relational schema

Entities

Department

Employee

SalaryGrade

Attribute questions

Is EmpNo an attribute If EmpNo already used in UoD, eg by an

existing system, then it is an attribute If it is not of interest to the user, and

just used to link tables, then ignore it

Attributes

DepartmentName

Location

EmployeeName

Job

AnnualSalary Commission

Hiredate

SalaryGrade

Hisal

Losal

Grade

Employs relationship

Meaning - department currently employs - note that this changes over time

Constraints every employee must work for exactly one

department a department may employ zero, one or

more employees Cardinality is 1 to 0.N

Relationships - Employs

DepartmentName

Location

EmployeeName

Job

AnnualSalary Commission

Hiredate

SalaryGrade

Hisal

Losal

Grade

employs

Manages relationship

Every employee except the top guy has exactly one manager

Employees manage zero, one or more employees

alsoa manager cannot manage

themselves - directly or indirectly

Relationship - Manages

DepartmentName

Location

EmployeeName

Job

AnnualSalary Commission

Hiredate

SalaryGrade

Hisal

Losal

Grade

employsmanages

Relationship Employee-grade

In the database this was ‘computed’ or ‘derived’ by finding the grade such that losal <= sal <= hisal

We can show such relationships in E-R (but more common in an Object model)

DepartmentName

Location

EmployeeName

Job

AnnualSalary Commission

Hiredate

SalaryGrade

Hisal

Losal

Grade

employsmanages

From UoD to E-R model

Which Entities?Entity - Attribute - Relationship

ambiguityEntity or Instance of EntityMany to Many relationships

Which Entities?

‘Nouns in description of Uod become entities in E-R’

Is Job an entityIs Location an entityIs information required to be held

about the term - details of responsibilities for Job, full address for Location.

Entity - Attribute - Relationship ambiguity

Suppose we need to know if an employee is married Attribute - married - True/False Attribute - name of spouse Relationship - married to (another

employee) Entity - marriage, with date of marriage,

date of divorce etc

Entity or Instance of Entity

Each Entity is a class of entity instances - employee entity has (14) entity

instancesBook / Copy problem

Book has a title, author, ISBN Copy has an accession no, location,

condition, onloanBook Copy

Many to Many relationships

Suppose employees move between departments, so that over a period of time an employee is employed by several departments

Department

Employee

employs

Department

Employee

Employment

Hiredate

Many to Many >> 2 One to Many

Next week

Lecture :Normalization Chapter 23

Tutorial : E-R modelling exercises