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Chapter 5

Understanding Entity Relationship

Diagrams



5-2

Outline

Notation basics

Understanding relationships

Generalization hierarchies Business rule representation

Diagram rules

Alternatie notations



5-3

Basic !ymbols

Entity "ype

symbol Relationship

symbol

#rimary $ey

Attributes

Relationship

name

Entity "ype

name

CourseNo

CrsDesc

CrsUnits

Course

O%%erNo

O%%&ocation

O%%"ime

Offering

'as



5-4

Cardinalities

Course Offering

Course( O%%ering(

Course)

Course*

O%%ering)

O%%ering*

O%%ering+



5-5

Cardinality Notation



5-6

Classi%ication o% Cardinalities

,inimum cardinality based ,andatory- e.istence dependent

Optional

,a.imum cardinality based /unctional

(0,

,0N (0(



5-7

!ummary o% Cardinalities



5-8

,ore Relationship E.amples

/ac!!N

/ac!alary

/acRan1

/ac'ireDate

Faculty

O%%erNo

O%%&ocation

O%%"ime

Offering

"eam"eachesO%%iceNo

O%%#hone

O%%"ype

Office

2or1s3n



5-9

Comparison to Access Notation

Course

Courseno

CrsDesc

CrsUnits

( 4

Offering

OfferNo

CourseNo

OffLocation

OffTime

...

Access Relationship Diagram



5-10

Understanding Relationships

3denti%ication dependency

,0N relationships ith attributes

!el% identi%ying relationships ,0ay relationships

E6uialence beteen ,0N and (0,

relationships



5-11

3denti%ication Dependency

Bldg3DBldgName

Bldg&ocation

Building

RoomNo

RoomCapacity

Roo

Contains

3denti%ication Dependency !ymbols-

!olid relationship line %or identi%ying

relationships

Diagonal lines in the corners denote5ea1 entities7



5-12

,0N Relationships

ith Attributes



5-13

,0N Relationships

ith Attributes 8339



5-14

3nstance Diagrams %or !el%0

Re%erencing Relationships/aculty(

/aculty) /aculty*

/aculty+ /aculty5

3!*::

3!*):

3!+4: 3!+;:

3!+;(

8a9 !uperises 8b9 #reRe6"o



5-15

ERD Notation %or

!el%0Re%erencing Relationships



5-16

Associatie Entity "ypes %or

,0ay Relationships

#artNo

#artName

"art

!uppNo

!uppName

#u$$lier

Associatie

entity type

#ro<No

#ro<Name

"ro%ect

&ses#art0Uses

!upp0Uses

#ro<0Uses



5-17

Relationship E6uialence

Replace ,0N relationship

Associatie entity type

"o identi%ying (0, relationships

,0N relationship ersus associatie entity

type

&argely pre%erence

Associatie entity type is more %le.ible in some

situations



5-18

Associatie Entity "ype E.ample

!td!!N

!tdName

#tudentO%%erNo

O%%&ocation

O%%"ime

Offering

EnrGrade

'nrollentRegisters

Grants AttDate

#resent

(ttendance

Recorded/or



5-19

Generalization 'ierarchies

Employee

EmpNo

EmpName

EmpHireDate

...

SalaryEmp

EmpSalary

HourlyEmp

EmpRate

generalization hierarchysymbol

supertype

subtypes



5-20

3nheritance

!ubtypes inherit attributes o% supertypes

8direct and indirect9

Allos abbreiation o% attribute list

Applies to code 8methods9 as ell as

attributes 8data9



5-21

Generalization Constraints



5-22

,ultiple &eels o%

Generalization



5-23

Comprehensie E.ample



5-24

Business Rules

En%orce organizational policies

#romote e%%icient communication

/ormal representation in ERD

3n%ormal representation in documentation

associated ith an ERD

Use rules language to %ormally representin relational database a%ter conersion



5-25

/ormal Representation

#rimary 1ey constraints- entity identi%ication

Named relationships- direct connections among

business entities

3denti%ication dependency- 1noledge o% otherentities %or identi%ication

Cardinalities- restrict number o% related entities

in a business situation

Generalization hierarchies- classi%ication o%

business entities and organizational policies



5-26

3n%ormal Representation

!peci%y as documentation associated elements

o% an ERD

Candidate 1ey constraints- alternate ays to

identi%y business entities Reasonable alues- %i.ed collection o% alues or

consistent ith another attribute

Null alue constraints- data collection

completeness

De%ault alues- simpli%y data entry and proide

alue hen un1non



5-27

Diagram Rules

Ensure that ERD notation is correctly used

!imilar to synta. rules %or a computer

language

Completeness rules- no missing

speci%ications

Consistency rules- no con%licts among

speci%ications

!upported by the ER Assistant



5-28

Completeness Rules

#rimary $ey Rule- all entity types hae a #$ 8direct=indirect= or inherited9

Naming Rule- all entity types= relationships= andattr ibutes hae a name

Cardinality Rule- cardinality is speci%ied in both directions%or each relationship

Entity #articipation Rule- all entity types participate in anat least one relationship e.cept %or entity types in ageneralization hierarchy

Generalization 'ierarchy #articipation Rule- at least oneentity type in a generalization hierarchy participates in arelationship



5-29

#rimary $ey Rule 3ssue

#rimary 1ey rule is simple in most cases

/or some ea1 entities= the #$ rule issubtle

2ea1 entity ith only one (0, identi%yingrelationship

2ea1 entity must hae a local 1ey to augment

the borroed #$ %rom the parent entity type

>iolation o% #$ rule i% local 1ey is missing



5-30

#$ Rule >iolation E.ample



5-31

Naming Consistency Rules

Entity Name Rule- entity type names must

be uni6ue

Attribute Name Rule- attribute names mustbe uni6ue ithin each entity type and

relationship

3nherited Attribute Rule- attribute names ina subtype do not match inherited 8direct or

indirect9 attribute names7



5-32

Relationship Names

No uni6ueness re6uirement

#articipating entities proide a conte.t %or

relationship names Use uni6ue names as much as possible to

distinguish relationships

,ust proide uni6ue names %or multiple

relationships beteen the same entity

types



5-33

Connection Consistency Rules

Relationship?Entity Connection Rule-

relationships connect to entity types 8not

necessarily distinct9

Relationship?Relationship Connection

Rule- relationships are not connected to

other relationships

Redundant /oreign $ey Rule- %oreign 1eys

are not used7



5-34

3denti%ication Dependency Rules

2ea1 entity rule- ea1 entities hae atleast one identi%ying relationship

3denti%ying relationship rule- at least one

participating entity type must be ea1 %oreach identi%ying relationship

3denti%ication dependency cardinality rule-

the minimum and ma.imum cardinalitymust e6ual ( %or a ea1 entity in allidenti%ying relationships



5-35

E.ample o% Diagram Errors



5-36

Corrected ERD



5-37

!upport in the ER Assistant

Relationship %ormation rules are supported

by diagram construction

Other rules are supported by the Chec1Diagram %eature /or the Redundant /oreign $ey rule= the

ER Assistant detects /$s that hae the

same name as the associated #$s



5-38

ERD >ariations

No standard ERD notation

!ymbol ariations

#lacement o% cardinality symbols

Rule ariations

Be prepared to ad<ust to the ERD notation

in use by each employer



5-39

ERD Rule >ariations

&ac1 o% ERD standards

,0ay relationships

,0N relationships

Relationships ith attributes

!el%0re%erencing relationships

Relationships connected to otherrelationships

Adapt to notations in or1 enironments



5-40

Chen ERD Notation



5-41

Uni%ied ,odeling &anguage

!tandard notation %or ob<ect0oriented

modeling

Ob<ects

Ob<ect %eatures

3nteractions among ob<ects

U,& supports class diagrams= inter%ace

diagrams= and interaction diagrams ,ore comple. than ERD notation



5-42

!imple Class Diagram



5-43

Association Class



5-44

Generalization Relationship



5-45

Composition Relationship



!ummary

Data modeling is an important s1ill

Cro@s /oot ERD notation is idely used

Use notation precisely

Use the diagram rules to ensure structural

consistency and completeness

Understanding the ERD notation is aprere6uisite to applying the notation on

business problems

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