Entity Relationship Model - WordPress.com · 4/4/2016 · Entity Relationship Model ... •Document analysis ... One-to-one binary relationship An entity instance in one entity type

Entity Relationship Model

การออกแบบโมเดลขอมล

Asst.Prof. Intiraporn Mulasastra

1

Objectives

Learn how to analyze real world data

Data and properties

Relationships

Represent data in a formal model, ER Model

The first step of database design

Learn to construct ER diagrams


2

Outline

Notation basics

Understanding entity relationships

Generalization hierarchies

Diagram rules


3

Data Modeling

Real World

Data Recordings

Data Modeling Data Representation

Data Items

<e,a,v>

<e,a,v>

<e,a,v>

The Notion of Data Physical Storage


Database Model


4

Database application development

1. Requirement analysis

2. Component design

1. Data modeling

2. SW design

3. Implementation

Requirement gathering

•User interview

•Document analysis

• reports, business rules, etc. •Stakeholder Meeting


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A Library Case

Collection of Books, index cards

Members

Borrowing and Returning books

Tom borrows a database book.

Tom is a member, noun

A database book is a book, noun

Borrowing is a process (verb) involving a book and a member


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Design stage

design


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Entity type (Entity class)

A collection of things of interest: persons, places, things, events, processes

Contains attributes with identifier(s) characteristics/properties of things (entities)

Offering

OfferNo

OffLocation OffTime

Course

CourseNum

CrsDesc CrsUnit

Entity type Entity type

attributes attributes


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Entity Instance

An entity

An entity instance is a specific

occurrence of an entity type. a member of an entity type


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Entity Instances

ID FirstName LastName GPA

5410545044 วรญญ ฤกษด 3.0

5410546334 วศน หาวาร 3.1

5610545013 นต เพชรรตนโมรา 3.2

5610545048 ธนธร อศวะอ านวย 3.3

Student

Entity type

4 entity instances

Members of a set must have the same type Student = {s1, s2, s3, s4}

attribute


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https://regis.ku.ac.th/query_course.php?TSm=2&TYr=57&qStd_id=5410545044&TCs_Code=01204351&TLec=450&TLab=0




Example


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Attributes

Properties of entity types

An entity type has attributes which together describe the entity

Student (ID, FirstName, LastName, GPA)

Each attribute has data type and other properties

Key or non-key


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Identifiers(keys)

• Entity instances have identifiers (keys)

• Keys are a type of attribute

Entity type : Student Contains attributes: ID, FirstName, LastName, GPA

Key : ID Unique value

ID can identify a specific instance in the entity type


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Types of Keys

Uniqueness

Keys may be unique or non-unique

If the key is unique, the data value for the key must be

unique among all instances of the entity

Candidate keys

Primary keys (one for each entity type)

Composite

A composite key consists of two or more attributes E.g., FirstName + LastName


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Is this blank sheet an entity type?

No attribute Asst.Prof. Intiraporn Mulasastra

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Is there a unique ATTRIBUTE ?

COLOR Asst.Prof. Intiraporn Mulasastra

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Entity Type vs. Entity Instance

An entity type is a description of the

structure and format of occurrences of

the entity

An entity instance is a specific occurrence of an entity type.


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Graphical presentation

•Entity type

•Entity type showing only the primary key attribute

•Entity type showing all attributes

•Conceptual design

•Preliminary design

•Final stage

1 2 3


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Relationships

Association among entity instances: have specific names

Usually between two entity types Unary

Binary

Ternary

etc. multiple entity types


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1.Unary Relationship

2.Binary Relationship

3.Ternary Relationship

teach


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Types of Binary Relationships

One to one

One to many

Many to one

Many to many


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One-to-One Relationship

Unary or binary

One-to-one binary relationship

An entity instance in one entity type is

related to an entity instance in another

entity type

Example

A student may have no

more than one email acct.

An email account may only

be used by one student


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One-to-Many Binary Relationship

Course

Offering การเปดสอน


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Bidirectional Relationship

◦ Can be used to navigate in both directions

◦ Two names Course to Offering: Has, Provides *

Offering to Course: IsProvidedFor

Which name to use: try to use active* verb; not always possible

Course

Offering

Ex. Course 351 has three offerings Asst.Prof. Intiraporn Mulasastra

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1:N relationship

ID Name

101 Database

102 OS

103 Network

OfferNo Semester Year

1 1 2014

2 2 2015

3 1 2015

ID Name

101 Database

102 OS

103 Network

1.Each course has many offerings

2.Each offering is provided for one course

Offering การเปดสอน

Course


1 1 2014

2 2 2015

3 1 2015

N:1 Offering to Course

1:N Course to Offering


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Many-to-Many Binary Relationship

Book

Member

enroll

borrow


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N:M relationship

ID Name

101 Sarah

102 Tom

103 Tim


1 1 2014

2 2 2015

3 1 2015

ID Name

101 Sarah

102 Tom

103 Tim

1.Each student can enroll in many offerings

2.Each offering can be enrolled by many students

Offering Student


1 1 2014

2 2 2015

3 1 2015


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How to represent relationships?

In E-R model, each entity type consists of its properties only !

Offering

OfferNo

OffLocation OffTime

Course

CourseNum

CrsDesc CrsUnit

Entity type Entity type

attributes of Course

attributes of Offering

Where is the relationship?


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Data Representation

Database models

e.g., Relational data model


Conceptual model

No physical data representation

Entity Relationship Diagram rectangles represents entity types and attributes

relationships represented by lines linking entities


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Data Validation


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Cardinality of Relationships

Cardinality means count

as a number of members in a set

Specify the number of entity instances that can

participate in a relationship instance

minimum : 0-n

maximum : 1- n

Course

Offering1 N

0,1,2,3,… or functional


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Questions about data validation

Course

Offering1:1 0:n

has

Asst.Prof. Intiraporn Mulasastra 32

E-R Diagram

Represent E-R model

Entity and attributes

a rectangle with entity name and attributes

underlined attribute is a primary key

Relationship

dashed line with a name

Cardinality

Offering

OfferNo

OffLocation OffTime

Course

CourseNum

CrsDesc CrsUnit

1:1 0:n

has


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Cardinality Notation (ERD)

A vertical line represents one (1)

A circle represents zero (0)

A crow’s foot represent many (N)

Any integers

Function names

Crow’s foot Asst.Prof. Intiraporn Mulasastra

34

Cardinality of Relationship Instances Minimum


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1:1 1:1

n:0 0:n

n:1 1:n

1:0 0:1

relationship ? ?


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Cardinality Notation

Inside symbol:

minimum cardinality

CourseNo

CrsDesc

CrsUnits

Course

OfferNo

OffLocation

OffTime

Offering

Has

Perpendicular line:

one cardinality

Outside symbol:

maximum cardinality

Circle: zero

cardinality

Crow's foot: many

cardinality

1:1(max:min) 0:n (min:max)


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Summary of Cardinalities

Classification Cardinality Restrictions

Mandatory Minimum cardinality 1

Optional Minimum cardinality = 0

Functional or single-

valued

Maximum cardinality = 1

1-M Maximum cardinality = 1 in

one direction and Maximum

cardinality > 1 in the other

direction.

M-N Maximum cardinality is > 1

in both directions.

1-1 Maximum cardinality = 1 in

both directions.


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More Relationship Examples

FacSSN

FacSalary

FacRank

FacHireDate

Faculty

OfferNo

OffLocation

OffTime

Offering

TeamTeachesOfficeNo

OffPhone

OffType

Office

WorksIn

WorksIn:

- 1-1

- Optional: office can be empty

- Mandatory: faculty must be

assigned to an office

TeamTeaches:

- M-N

- Optional in both

directions


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Teacher

Conferenceattend

Customer

Product? ?

purchase


Football Team

Player? ?

play

Depend on business rules Asst.Prof. Intiraporn Mulasastra

40

Bank Branch

Account? ?

has

Product

Warehouse? ?

is-in



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More on Relationships

Next week


42

Understanding Relationships

1. Identification dependency

2. M-N relationships with attributes

3. Self identifying relationships

4. M-way relationships

5. Equivalence between M-N and 1-M

relationships


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1.Identification Dependency

Identification dependency involves existence

dependency:

Weak entity is existent dependent on other entity

Also borrows part or all of PK

Ex. Existence of a room

Building

Room


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Identification Dependency

BldgID

BldgName

BldgLocation

Building

RoomNo

RoomCapacity

Room

Contains

Identification Dependency Symbols:

Solid relationship line for identifying

relationships

Diagonal lines in the corners denote

weak entities.

Others: Province & district, order-orderline

Indicates the source of PK

Borrows part or all of PK


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M-N Relationships

with Attributes

StdSSN

StdName

StudentOfferNo

OffLocation

OffTime

Offering

EnrollsIn

EnrGrade

attribute of relationship


46

M-N Relationships

with Attributes (II)

PartNo

PartName

Part

SuppNo

SuppName

Supplier

Qty

Provides

a) Provides relationship

AuthNo

AuthName

Author

ISBN

Title

Book

AuthOrder

b) Writes relationship

Writes


47

Instance Diagrams for

Self-Referencing Relationships

Faculty1

Faculty2 Faculty3

Faculty4 Faculty5

IS300

IS320

IS480 IS460

IS461

(a) Supervises (b) PreReqTo


48

ERD Notation for

Self-Referencing Relationships

FacSSN

FacName

Faculty

a) manager-subordinate

Supervises

b) course prerequisites

CourseNo

CrsDesc

Course PrereqTo


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M-way Relationships


50

Associative Entity Types for

M-way Relationships

PartNo

PartName

Part

SuppNo

SuppName

Supplier

Associative

entity type

ProjNo

ProjName

Project

UsesPart-Uses

Supp-Uses

Proj-Uses

3 way relationship tracks who supplies a part on a specified project Asst.Prof. Intiraporn Mulasastra

51

Example

Project Part Supplier

101 Door HomePro 101 Door ThaiPro 102 Window HomePro 102 Door ThaiPro

Uses


52

Relationship Equivalence

Replace M-N relationship

Associative entity type

Two identifying 1-M relationships

M-N relationship versus associative entity type

Largely preference

Associative entity type is more flexible in some situations


53

Associative Entity Type Example

StdSSN

StdName

StudentOfferNo

OffLocation

OffTime

Offering

EnrGrade

EnrollmentRegistersGrants

AttDate

Present

Attendance

RecordedFor

Attendance: - Weak entity - PK: Combination of AttDate and PK of Enrollment Must use associative entity type for Enrollment rather than M-N relationship


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Generalization Specialization


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Generalization

A bottom-up design process – combine a number of

entity sets that share the same features into a higher-

level entity set.

Specialization and generalization are simple inversions

of each other; they are represented in an E-R diagram

in the same way.

The terms specialization and generalization are used

interchangeably.


56

The ISA relationship also referred to as

superclass - subclass relationship

ISA

ISA


57

Generalization Hierarchies

Employee

EmpNo

EmpName

EmpHireDate

...

SalaryEmp

EmpSalary

HourlyEmp

EmpRate

generalization hierarchy

symbol

supertype

subtypes


58

Example

Person

Employee Customer

Asst.Prof. Intiraporn Mulasastra 59

Inheritance

Subtypes inherit attributes of supertypes (direct and

indirect)

Allows abbreviation of attribute list

Applies to code (methods) as well as attributes

(data)

Reduce the amount of code by inheriting code from similar objects


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Multiple Levels of Generalization

Security

Symbol

SecName

LastClose

Stock

OutShares

IssuedShares

Bond

Rate

FaceValue

D,C

Common

PERatio

Dividend

Preferred

CallPrice

Arrears

D,C

หนบรมสทธ หนสามญ

หลกทรพย

พนธบตร หนทน


61

Comprehensive Example

Offering

OfferNo

OffLocation

OffTime

EnrGrade

EnrollmentRegisters

Grants

CourseNo

CrsDesc

CrsUnits

Course

Faculty

FacSalary

FacRank

FacHireHate

SSN

Name

City

State

Zip

UnivPerson

Has

Teaches

Supervises

C

Student

StdClass

StdMajor

StdGPA

A faculty member can be a student too.


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Diagram Rules

Ensure that ERD notation is correctly used

Similar to syntax rules for a computer language

Completeness rules: no missing specifications

Consistency rules: no conflicts among

specifications


63

PK Rule Violation Example

BldgID

BldgName

BldgLocation

BuildingRoom

RoomNo

RoomCapacityContains

PK rule violation

A single 1-M identifying relationship

Room does not have a local key.

RoomNo is not

underlined No Local key


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Example

Offering

OfferNo

OffLocation

OffTime

EnrGrade

EnrollmentRegisters

Grants

CourseNo

CrsDesc

CrsUnits

Course

Faculty

FacSalary

FacRank

FacHireHate

SSN

Name

City

State

Zip

UnivPerson

Has

Teaches

Supervises

C

Student

StdClass

StdMajor

StdGPA

Why not a weak entity


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Summary

Data modeling is an important skill

Crow’s Foot ERD notation is widely used

Use notation precisely

Use the diagram rules to ensure structural

consistency and completeness

Understanding the ERD notation is a prerequisite

to applying the notation on business problems


66

Documents

Entity Relationship Model - WordPress.com · 4/4/2016 · Entity Relationship Model ... •Document analysis ... One-to-one binary relationship An entity instance in one entity type