CMPT 354:Database System I

Lecture 8. The E/R Model

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Motivation

• How to figure out this database design?

• What tables to create?• Which attributes should be added to each table?• What are the relationships between the tables?

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History of E/R Model

• E/R Model (Entity-RelationshipModeling)• Codd wrotealonglettercriticizingpaper• Many people suggested him to give up this idea

• Why not build RDBMS based on E/R Model?• Noquerylanguage proposed• RelationalDBMSinthe1970’s

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Dr. Peter Chen

Outline

• E/RBasics:Entities&Relationships

• E/RDesignconsiderations

• AdvancedE/RConcepts

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Outline

• E/RBasics:Entities&Relationships• DatabaseDesign• Entities/Entitysets/Keys/Relationships

• E/RDesignconsiderations

• AdvancedE/RConcepts

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DatabaseDesign

• Databasedesign:Whydoweneedit?• Agreeonstructureofthedatabasebeforedecidingonaparticularimplementation

• Considerissuessuchas:• Whatentitiestomodel• Howentitiesarerelated• Whatconstraintsexistinthedomain• Howtoachievegood designs

• Severalformalismsexist• WediscussoneflavorofE/Rdiagrams

DatabaseDesignProcess

1. Requirementsanalysis

• Whatdataisgoingtobestored?

• Whatarewegoingtodowiththedata?

• Whoshouldaccessthedata?

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1.RequirementsAnalysis 2.ConceptualDesign 3.Logical,Physical,Security,etc.

Technicalandnon-technicalpeopleareinvolved

2.ConceptualDesign

• Ahigh-leveldescription ofthedatabase

• Sufficientlyprecise thattechnicalpeoplecanunderstandit

• But,notsoprecisethatnon-technicalpeoplecan’tparticipate

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1.RequirementsAnalysis 2.ConceptualDesign 3.Logical,Physical,Security,etc.

DatabaseDesignProcess

ThisiswhereE/Rfitsin.

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3.More:

• LogicalDatabaseDesign

• PhysicalDatabaseDesign

• SecurityDesign

1.RequirementsAnalysis 2.ConceptualDesign 3.Logical,Physical,Security,etc.

DatabaseDesignProcess

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E/RisavisualsyntaxforDBdesignwhichispreciseenough fortechnicalpoints,butabstractedenough fornon-technicalpeople

MakesProduct

name category

price

Company

name

E/RModel&Diagramsused

1.RequirementsAnalysis 2.ConceptualDesign 3.Logical,Physical,Security,etc.

DatabaseDesignProcess

EntitiesandEntitySets• Anentity isanindividualobject• Eg:Aspecificpersonorproduct

• Anentityset isacollectionofentitiesofthesametype• ThesearewhatisshowninE/Rdiagrams- asrectangles• Eg:Person,Product

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ProductPerson

Attributes

• Anentitysethasattributes• Representedbyovalsattachedtoanentityset

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Product

name category

price

Example

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Product

name category

price

EntitySet

Product

Name:XboxCategory:Total

MultimediaSystemPrice:$250

Name:MyLittlePonyDollCategory:ToyPrice:$25

Entity

EntityAttribute

Entitiesarenot explicitlyrepresentedinE/Rdiagrams!

Keys

• Akey isasetofattributesthatuniquelyidentifiesanentity.• Everyentitysetmusthaveakey

• Denoteelementsoftheprimarykeybyunderlining.

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Product

name category

price

TheRinE/R:Relationships

• Arelationship isbetweentwoentities

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Product

name category

priceCompany

name

Makes

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makes

buys employs

Product

name category

price

Person

address name ssn

Company

stockprice

name

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WhatisaRelationship?

• Amathematicaldefinition:

• LetA,Bbesets• A={1,2,3},B={a,b,c,d}

1

2

3

a

b

c

d

A= B=

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WhatisaRelationship?• Amathematicaldefinition:

• LetA,Bbesets• A={1,2,3},B={a,b,c,d}

• AxB(thecross-product)isthesetofallpairs(a,b)• A´ B={(1,a),(1,b),(1,c),(1,d),(2,a),(2,b),(2,c),(2,d),(3,a),(3,b),(3,c),(3,d)}

1

2

3

a

b

c

d

A= B=

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WhatisaRelationship?

• Amathematicaldefinition:

• LetA,Bbesets• A={1,2,3},B={a,b,c,d},

• AxB(thecross-product)isthesetofallpairs(a,b)• A´ B={(1,a),(1,b),(1,c),(1,d),(2,a),(2,b),(2,c),(2,d),(3,a),(3,b),(3,c),(3,d)}

• Wedefinearelationship tobeasubsetofAxB• R={(1,a),(2,c),(2,d),(3,b)}

1

2

3

a

b

c

d

A= B=

WhatisaRelationship?

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name category priceiPhone8 Electronics $700iPad4 Electronics $300Office Software $120

nameAppleMicrosoft

ProductCompany

MakesProduct

name category

price

Company

name

Arelationship betweenentitysetsPandCisasubsetofallpossiblepairsofentitiesinPandC,withtuplesuniquelyidentifiedbyPandC’skeys

WhatisaRelationship?

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name category priceiPhone8 Electronics $700iPad4 Electronics $300Office Software $120

nameAppleMicrosoft

ProductCompanyC.name P.name P.category P.priceApple iPhone8 Electronics $700Apple iPad4 Electronics $300Apple Office Toys $120Microsoft iPhone8 Electronics $700Microsoft iPad4 Electronics $300Microsoft Office Toys $120

CompanyC×ProductP

MakesProduct

name category

price

Company

name

Arelationship betweenentitysetsPandCisasubsetofallpossiblepairsofentitiesinPandC,withtuplesuniquelyidentifiedbyPandC’skeys

WhatisaRelationship?

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C.name P.name

Apple iPhone8

Apple iPad4

Microsoft Office

Makes

name category priceiPhone8 Electronics $700iPad4 Electronics $300Office Software $120

nameAppleMicrosoft

ProductCompanyC.name P.name P.category P.priceApple iPhone8 Electronics $700Apple iPad4 Electronics $300Apple Office Software $120Microsoft iPhone8 Electronics $700Microsoft iPad4 Electronics $300Microsoft Office Software $120

CompanyC×ProductP

MakesProduct

name category

price

Company

name

Arelationship betweenentitysetsPandCisasubsetofallpossiblepairsofentitiesinPandC,withtuplesuniquelyidentifiedbyPandC’skeys

WhatisaRelationship?

• Therecanonlybeonerelationshipforeveryuniquecombinationofentities

• Thisalsomeansthattherelationshipisuniquelydeterminedbythekeysofitsentities

• Example:the“key”forMakes(toright)is{Product.name,Company.name}

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Thisfollowsfromourmathematicaldefinitionofarelationship- it’saSET!

MakesProduct

name category

price

Company

name

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Product

name category

priceCompany

name

Makes

since

• Relationshipsmayhaveattributesaswell.

Forexample:“since”recordswhencompanystartedmakingaproduct

RelationshipsandAttributes

C.name P.name Since

Apple iPhone8 2018.09.01

Apple iPhone8 2017.09.01

Makes

Decision:Relationshipvs.Entity?

• Q:Whatdoesthissay?

• A:Apersoncanonlybuyaspecificproductonce

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PurchasedProduct

name category

price

Person

name

date

Person.name Product.name Date

Jiannan iPhone8 2018.10.01

Jiannan iPhone8 2018.12.01

Purchase

Decision:Relationshipvs.Entity?

• Whataboutthisway?

• Nowwecanhavemultiplepurchasesperproduct,personpair!

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Product

name category

price

Person

name

date

Purchase

quantityPID#

ProductOf BuyerOf

Wecanalwaysuseanewentityinsteadofarelationship.Forexample,topermitmultipleinstancesofeachentitycombination!

Exercise-1

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DrawanE/Rdiagramforgeography

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Entities• Country:name,area,population,gdp• City:name,population,longitude,latitude• River:name,length• Sea:name,maxdepth

Relationships• CitybelongstoCountry• RivercrossesCountry• RiverendsinSea

Outline

• E/RBasics:Entities&Relationships• DatabaseDesign• Entities/Entitysets/Keys/Relationships

• E/RDesignconsiderations• Relationshipscond’s:multiplicity,multi-way• Designconsiderations• ConversiontoSQL

• AdvancedE/RConcepts

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MultiplicityofE/RRelationships

Indicatedusingarrows

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abcd

One-to-one:

123

abcd

Many-to-one:

123

abcd

One-to-many:

123

abcd

Many-to-many:

X->YmeansthereexistsafunctionmappingfromXtoY(recallthedefinitionofafunction)

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address name ssn

Person

buys

makes

employs

CompanyProduct

name category

stockprice

name

price

Whatdoesthissay?

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Multi-wayRelationships

Howdowemodelapurchaserelationshipbetweenbuyers,productsandstores?

Purchase

Product

Person

Store

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Q:Whatdoesthearrowmean?

ArrowsinMultiwayRelationships

Purchase

Product

Person

Store

givenaperson,candeterminewhattheyboughtandthestorewheretheyboughtit

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Q:Whatdoesthearrowmean?

ArrowsinMultiwayRelationships

Purchase

Product

Person

Store

givenastore,candeterminewhoshoppedthereandtheproducttheyboughteachstoresellsoneproductandtooneperson,ever

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Q:Howdowesaythateverypersonshopsinatmostonestore?

ArrowsinMultiwayRelationships

A:Cannot.Thisisthebestapproximation.(Whyonlyapproximation?)

Product

Person

StorePurchase

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ConvertingMulti-wayRelationshipstoBinary

Purchase

Person

Store

Product

StoreOf

ProductOf

BuyerOf

date Fromwhatwehadonpreviousslidetothis- whatdidwedo?

Decision:Multi-wayorNewEntity+Binary?

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Purchase

Person

Store

Product

StoreOf

ProductOf

BuyerOf

date

Purchase

Product

Person

Store

Multi-wayRelationship Entity+Binary

• (B)isalsousefulwhenwewanttoadddetails(constraintsorattributes)totherelationship- “Apersonwhoshopsinatmostonestore”- “Howlongapersonhasbeenshoppingatastore”

• (A)isusefulwhenarelationshipreallyisbetweenmultipleentities- Ex:Athree-partylegalcontract

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DesignPrinciples

PurchaseProduct Person

What’swrongwiththeseexamples?

President PersonCountry

multiplepresidents,alsomaywanttorequirecountrytohavepresident

productbuysonlyoneproduct,thenout

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DesignPrinciples:What’sWrong?

Purchase

Product

Store

date

personNamepersonAddr

maybepeopleshouldbeentities!

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Purchase

Product

Person

Store

dateDates

DesignPrinciples:What’sWrong?

datesdon’tneedtobeanentitybythemselves

Examples:Entityvs.Attribute

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Shouldaddress(A)beanattribute?

Employee

Addr 1 Addr 2

Address

StreetAddr ZIP

Employee

AddrOf

Or(B)beanentity?

Examples:Entityvs.Attribute

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Shouldaddress(A)beanattribute?

Howdowehandleemployeeswithmultipleaddresseshere?

Howdowehandleaddresseswhereinternalstructureoftheaddress(e.g.zipcode,state)isuseful?Employee

Addr 1 Addr 2

Examples:Entityvs.Attribute

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Shouldaddress(A)beanattribute?

Employee

Addr 1 Addr 2

Address

StreetAddr ZIP

Employee

AddrOf

Or(B)beanentity?

Ingeneral,whenwewanttorecordseveralvalues,wechoosenewentity

Exercise-2

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DrawanE/Rdiagramforgeography

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Entities• Country:name,area,population,gdp• City:name,population,longitude,latitude• River:name,length• Sea:name,maxdepth

Relationships• Eachcitybelongstoasinglecountry• Eachrivercrossesoneorseveralcountries• Eachriverendsinasinglesea

FromE/RDiagramstoRelationalSchema• Keyconcept:

BothEntitysetsandRelationships becomerelations(tablesinRDBMS)

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FromE/RDiagramstoRelationalSchema

name price category

iPhone 700 Electronics

Office 150 Software

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Product

price category

name

Product

• Anentitysetbecomesatable– Eachrowisoneentity– Eachrowiscomposedoftheentity’s

attributes,andhasthesameprimarykey

CREATE TABLE Product(name CHAR(50) PRIMARY KEY,price DOUBLE,category VARCHAR(30)

)

FromE/RDiagramstoRelationalSchema

name firstname lastname date

iPhone Mike Jordan 01/01/18

iPhone Jiannan Wang 01/03/18

iPad John Smith 01/05/18

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Purchased

• Arelationshipalsobecomesatable– AddPrimaryKey– AddForeignKey

PurchasedProduct

name category

pricePerson

firstnamedate lastname

CREATE TABLE Purchased(name CHAR(50),firstname CHAR(50),lastname CHAR(50),date DATE,PRIMARY KEY (name, firstname, lastname),FOREIGN KEY (name)

REFERENCES Product,FOREIGN KEY (firstname, lastname)

REFERENCES Person)

Exercise-3

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FromE/RDiagramtoRelationalSchema

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Howdowerepresentthisasarelationalschema?

Outline

• E/RBasics:Entities&Relationships• DatabaseDesign• Entities/Entitysets/Keys/Relationships

• E/RDesignconsiderations• Relationshipscond’s:multiplicity,multi-way• Designconsiderations• ConversiontoSQL

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makes CompanyProduct

name categoryname

price

country

Outline

• E/RBasics:Entities&Relationships• DatabaseDesign• Entities/Entitysets/Keys/Relationships

• E/RDesignconsiderations• Relationshipscond’s:multiplicity,multi-way• Designconsiderations• ConversiontoSQL

• AdvancedE/RConcepts• Combing Relations• Constraints• Subclass• Weak Entity Sets

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• For many-to-one relationships

Combing Relations

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makes CompanyProduct

name categoryname

price

P.name C.name

iPhone8 Apple

iPad4 Apple

Office Microsoft

name category priceiPhone8 Electronics $700iPad4 Electronics $300Office Software $120

name countryApple USMicrosoft US

Product MakeCompany

country

Combing Relations

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• Remember:noseparaterelationsformany-onerelationship

P.name C.name category priceiPhone8 Apple Electronics $700iPad4 Apple Electronics $300Office Microsoft Software $120

name category priceiPhone8 Electronics $700iPad4 Electronics $300Office Software $120

name countryApple USMicrosoft US

Product MakeCompany

name countryApple USMicrosoft US

P.name C.name

iPhone8 Apple

iPad4 Apple

Office Microsoft

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ConstraintsinE/RDiagrams

• FindingconstraintsispartoftheE/Rmodelingprocess.Commonlyusedconstraintsare:

• Keys• Ex:A product name uniquelyidentifiesaproduct

• Single-valueconstraints:• Ex:aproduct made by exactly one company

• Participation constraints:• Ex:all products are made by a company

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KeysinE/RDiagrams

address name ssn

Person

Product

name category

price

Underlinekeys:

Note:noformalwaytospecifymultiple keysinE/Rdiagrams…

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Single-Value Constraints

CompanyProduct makes

CompanyProduct makes

Eachproductmadebyatmostonecompany.Someproductsmadebynocompany?

Eachproductmadebyexactly onecompany.

ParticipationConstraints:Partialv.Total

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makesProduct Company

Arethereproductsmadebynocompany?Companiesthatdon’tmakeaproduct?

makesProduct Company

Boldlineindicatestotalparticipation (i.e.here:allproductsaremadebyacompany)

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ModelingSubclasses

Someobjectsinaclassmaybespecial• Defineanewclass?

• Butwhatifwewanttomaintainconnectiontocurrentclass?

• Better:defineasubclass• Ex:

Products

Softwareproducts

Hardwareproducts

Wecandefinesubclasses inE/R!

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Product

name

price

SoftwareProduct

platforms

Hardware Product

weight

isA

ModelingSubclasses

Childsubclassescontainalltheattributesofalloftheirparentclassesplus thenewattributesshownattachedtothemintheE/Rdiagram

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UnderstandingSubclasses

• Thinkintermsofrecords;ex:

• Product

• SoftwareProduct

• HardwareProduct

nameprice

namepriceplatforms

namepriceweight

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name price

iphone 8 700

iPad 4 300

office 100

name platforms

office windows

name weight

iphone 8 148 g

ipad 4 650 g

Product

SoftwareProduct

HardwareProduct

SubclassestoRelations

IsA Review

• IfwedeclareAIsA BtheneveryA isaB

• WeuseIsA to

• Adddescriptiveattributestoasubclass

• Toidentifyentitiesthatparticipateinarelationship

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ModelingUnionTypes WithSubclasses

Say: eachpieceoffurnitureisownedeitherbyaperson,orbyacompany

FurniturePiecePerson Company

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ModelingUnionTypes WithSubclasses

Say:eachpieceoffurnitureisownedeitherbyapersonorbyacompany

Solution1. Acceptable,butimperfect(What’swrong?)

FurniturePiecePerson Company

ownedByPerson ownedByComp

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ModelingUnionTypes WithSubclasses

Solution2:better(thoughmorelaborious)

FurniturePiece

Person Company

ownedBy

Owner

isa

WeakEntitySets

Entitysetsareweak whentheirkeycomesfromotherclassestowhichtheyarerelated.

“Introduction to database”vs.“TheSFU introduction to database”

uname

UniversityCourse Offer

cnametextbook

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WeakEntitySets

• cname isapartialkey (denotewithdashedunderline).• Universityiscalledthesupporting entity set• Offer is called the supporting relationship

uname

UniversityCourse Offer

cnametextbook

Entitysetsareweak whentheirkeycomesfromotherclassestowhichtheyarerelated.

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WeakEntitySets to Relations

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uname

UniversityCourse Offer

cnametextbook

Course(cname, uname, textbook)

University(uname)

Offering(cname, Course.uname, University.uname)

E/R Summary

• E/RBasics:Entities&Relationships• DatabaseDesign• Entities/EntitySets/Keys/Relationships

• E/RDesignconsiderations• Relationshipscond’s:multiplicity,multi-way• Designconsiderations• ConversiontoSQL

• AdvancedE/RConcepts• Combing Relations• Constraints• Subclass• Weak Entity Sets

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Acknowledge• Some lecture slides were copied from or inspired by thefollowing course materials• “W4111: Introduction to databases” by Eugene Wu atColumbia University• “CSE344: IntroductiontoDataManagement” by Dan Suciu atUniversityof Washington• “CMPT354: Database System I” by JohnEdgar at Simon FraserUniversity• “CS186: Introduction to Database Systems” by Joe Hellersteinat UC Berkeley• “CS145: IntroductiontoDatabases” by Peter Bailis at Stanford• “CS348:IntroductiontoDatabaseManagement” by GrantWeddell at University of Waterloo

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