Database Design

J.G. ZhengMay 19th 2008

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Overview

Entity Relationship Modeling Data modeling using Entity

Relationship Diagram (ERD)

Transforming ERD into tables

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Introduction

Design is a process Analysis

Requirements (user view) Conceptual modeling (ERD)

(Logical) design Transforming ERD to tables and keys

Physical design and implementation Using MS Access

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Entity-Relationship Model

A model to represent the real world

Elements Entities Attributes Identifiers Relationships

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Entity

Entity Anything in the real world

Entity class (entity set) A set of things of the same type

Entity instance An exact instance of an entity class

ERD notation for entity A rectangular box

Books

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Attribute and Identifier

Attribute Properties of entity Notation: an oval

Identifier An attribute with unique value to identify each instance

Title

Books

ISBN

Price

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Relationship

Notation: a diamond

Degree of relationship Unary (recursive) – only 1 entity Binary – 2 entities Ternary – 3 entities N-ary – more entities

Books Publisherspublish

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Types of Binary Relationship

One-to-One (1:1) A single entity instance in one entity class is

related to a single entity instance in another entity class

Examples: Governors : States Students : PantherCards

ER Notation (Crow’s foot)

Governors Statesgovern

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Types of Binary Relationship

One-to-Many (1:N) A single entity instance in one entity class

(parent) is related to multiple entity instances in another entity class (child)

Examples Customers : Orders Students : Degrees

ER Notation (Crow’s foot)

Books Publisherspublish

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Types of Binary Relationship

Many-to-Many (N:M) Each entity instance in one entity class is

related to multiple entity instances in another entity class; and vice versa

Examples Books : Writers Customers : Banking Accounts

ER Notation (Crow’s foot)

Books Authorswrite

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A Complete ER Example

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ER Diagram ExerciseDraw a ERD about movie data

Let’s only consider the following entities and their attributes

Perfomers (Actors/Actresses): PerformerID, FirstName, LastName, Gender

Movies: MovieID, Title, Maker, Year MovieMakers (companies): MakerID, Name

Assumptions A movie one or more actors/actresses An actor/actress can be in multiple movies A movie is made by only one company

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Transforming ER to Tables

Guidelines to create tables Each entity becomes a relation (table) Attributes of the entity become fields of that

table Identifier becomes primary key

Rules to transform relationships Based on the type of the relationship

One-to-One One-to-Many Many-to-Many

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Transforming 1:1 Relationships

One-to-One The key from one relation is placed in

the other as a foreign key

LockerDesc

Location

LockerID

Locker

LockerID

EmpName

EmpID

Employee

Foreign Key

Primary Key

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Transforming 1:1 Relationships

EmpID

LockerDesc

Location

LockerID

Locker

EmpName

EmpID

Employee

Foreign Key

Primary Key

Usually, it does not matter which table receives the foreign key

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Transforming 1:N Relationships

One-to-Many The primary key from the “One” side is placed

in the “Many” side as a foreign key The foreign key is always on the “Many” side

Instructor

InstructorID

FirstName

Office

Department

CourseSection

CRN

Semester

CourseID

Time

TaughtBy

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Transforming 1:N Relationships

Another example

DeptName

Location

DeptID

Department

Dept

EmpName

EmpID

Employee

Foreign Key

Primary Key

Department Employeehas

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Transforming N:M Relationships

Many-to-Many There is no direct way to map many

to many relationships To represent an M:N relationship, a

table is created (intersection table) for the relationship

Orders PartsOrderItems

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Transforming N:M Example

Many-to-Many is designed as two One-to-Many relationships

Create an intersection table with the primary key from each original table as foreign keys

The intersection table usually has a composite primary key

OrderItems

OrderNum

PartNum

NumOrdered

Orders

OrderNum

OrderDate

CustomerNum

Parts

PartNum

Description

Class

Price

1:N

N:1

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M:N Relationship Exercise

Performers MoviesCast

PerformerId

FirstName

LastName

Length

Title

Character

MovieId

Gender

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Review QuestionGiven the entities SUPPLIER and PRODUCT are one-to-many relationship, which of the following would represent the correct table design?

A. PRODUCT (ProductID, Description, Cost)SUPPLIER (SupplierID, ContactName, PhoneNumber)

FK ProductID PRODUCT

B. PRODUCT (ProductID, Description, Cost, SupplierID)FK SupplierID SUPPLIER

SUPPLIER (SupplierID, ContactName, PhoneNumber, ProductID)FK ProductID PRODUCT

C. PRODUCT (ProductID, Description, Cost, ContactName) FK ContactName SUPPLIER SUPPLIER (SupplierID, ContactName, PhoneNumber)

D. PRODUCT (ProductID, Description, Cost, SupplierID)FK SupplierID SUPPLIER

SUPPLIER (SupplierID, ContactName, PhoneNumber)

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Summary

E-R diagram is used widely for database design

Remember the rules to transform ERD to actual tables and relationships One to one One to many Many to many