DatabaseManagementSystem

Lecture3

DatabaseDesign– ERModelandERmodeltoRelationalSchemas

*SomematerialsadaptedfromR.Ramakrishnan,J.Gehrke andShawnBowers

Today’sAgenda• ERModel

• ERModeltoRelational

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ERModel

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ConceptualDataModeling• Similartosoftwaredesign...• requirementsgatheringandanalysis

• applicationarchitectureanddesign

• implementationandtesting

• maintenance

• databasedesigninvolvesmultiplesteps(esp.priortocreatingtabledefinitions)

• HerewefocusonconceptualdesignusingtheEntity- RelationshipModel• similartotheuseofUMLdiagramsforsoftwaredesign

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Entity-Relationship(ER)Diagram[Chen71]

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Employee Department

Project

Home

Manager

Projectmember

Sponsor

EntitySet

RelationshipSet

Attribute

ssn name lot code name

number

name

startdate

enddate

budget

Key:

Terminology• An”Entity”isanobjectthatcanbedistinguishedfromotherobject• e.g.,theindividual“JohnSmith”oraparticularcompany

• describedusingasetofattribute-valuepairs

• oneormoreattributesasids(i.e.,keys)

• An“EntitySet”isacollectionofsimilarentities• e.g.,thesetofemployeeentities

• definedbythetypeofattributesandrelationshipsentites ofthesetarecharacterizedby

• ofteninformallycalledan“entity”ifeveryoneknowswearetalkingabouttheschema

• AnEntitySetisalsobecalledan“EntityType”

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Terminology• A“Relationship”isanassociationamong2ormoreentities• e.g.,John’shomedepartmentisPharmacy2

• justasentitiesareinstancesofentitysets,relationshipsare

• instancesofrelationshipsets...

• A“RelationshipSet”isacollectionofsimilarrelationships• e.g.,thesetofhomedepartmentrelationships

• definedbytheparticipatingentitytypesandotherconstraints

• ofteninformallycalleda“relationship”ifeveryoneknowswearetalkingabouttheschema

• ARelationshipSetisalsobecalleda“RelationshipType”

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Entity-RelationshipModelvs.RelationalModel• Adifferentdatamodelthantherelationalmodel• differentconstructsformodelingschemasanddata

• DBMSsystems(invariousforms)haveevenbeenbuiltontheERmodel...thoughprimarilyusedasadesigntool

• Therelationalmodelhas:• Tables(relations)withattributes,primarykeys,foreignkeys,rows,values

• TheERmodelhas:• EntitiesandEntitySetswithattributesandentityidentifiers(likekeys)

• RelationshipsandRelationshipSetswithcardinalityconstraints,roles,attributes,etc.

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MappingERmodelstoRelationalSchemas

• Thismappingassumes:• Employeeshaveonehomedepartment

• Departmentshaveonemanager

• Employeescanparticipateinmanyprojects

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Employee(ssn,name,lot,home-dept)ProjectMember(ssn,number)Department(code,name,manager)Project(number,name,startdate,enddate,budget,sponsor)

ERCardinalityConstraints

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AnEmployeecanhave 0or1homeDepartment

Employee Department

Project

Home

Manager

Projectmember

Sponsor

ssn name lot code name

number

name

startdate

enddate

budget

0..1

ERCardinalityConstraints

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ADepartemnt canhave 0ormanyhomeEmployees

Employee Department

Project

Home

Manager

Projectmember

Sponsor

ssn name lot code name

number

name

startdate

enddate

budget

0..10..*

ERCardinalityConstraints

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ADepartmentmusthaveexactlyoneManager

Employee Department

Project

Home

Manager

Projectmember

Sponsor

ssn name lot code name

number

name

startdate

enddate

budget

0..10..*

1..1

ERCardinalityConstraints

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andsoon…

Employee Department

Project

Home

Manager

Projectmember

Sponsor

ssn name lot code name

number

name

startdate

enddate

budget

0..10..*

1..10..* 0..1 1..1

0..*

0..*

ERCardinalityConstraints• Whichoneiscorrect?

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Employee DepartmentHome0..11..*

Employee DepartmentHome0..10..*

Employee DepartmentHome1..*0..*

Mustdiscoverthesemanticsoftheapplications

ERCardinalityConstraints(1/5)• ConstraintsareexpressedoverEntitySetsandRelationshipSets

• ConstraintsonthemembersofthecorrespondingEntitiesandRelationships

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Employee DepartmentHome0..11..*

homerelationship

Doesthissatisfythecardinalityconstraints?

ERCardinalityConstraints (2/5)• ConstraintsareexpressedoverEntitySetsandRelationshipSets

• ConstraintsonthemembersofthecorrespondingEntitiesandRelationships

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Employee DepartmentHome0..11..*

homerelationship

Howaboutnow?

ERCardinalityConstraints (3/5)• ConstraintsareexpressedoverEntitySetsandRelationshipSets

• ConstraintsonthemembersofthecorrespondingEntitiesandRelationships

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Employee DepartmentHome0..11..*

homerelationship

Andnow?

ERCardinalityConstraints(4/5)• Therearevariousnotationsusedforwritingcardinalityconstraints...

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Examplesof“OnetoMany”constraints

1 n

one many

1 *

Maximumcardinalitiesonly

0:1 1:n

zero..one one..many

0:1 1:*

Maximumandmaximumcardinalities

0

0-1 1+

ERCardinalityConstraints(5/5)• Therearevariousnotationsusedforwritingcardinalityconstraints...

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Examplesof“ManytoMany”constraints

m n

many many

* *

Maximumcardinalitiesonly

1:n 1:n

one..many one..many

Maximumandmaximumcardinalities

1+ 1+

RelationshipSetAttributes(1/4)

• Eachinstanceoftherelationshiphasavaluefortheattribute...

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Home0..11..*

homerelationship

ssn name lot code name

Employee Department

startdate

RelationshipSetAttributes (2/4)

• Whichonemakessence?

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Home0..11..*

homerelationship

ssn name lot code name

Employee Department

startdate

startdate startdate

RelationshipSetAttributes (3/4)

• BecauseEmployeeshavezerooronehomedepartments,

• ...startdatewillworkasanEmpoyee orhomeattribute

• Whataboutstartdate atDepartment?

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Home0..11..*

homerelationship

ssn name lot code name

Employee Department

startdate

startdate startdate

RelationshipSetAttributes (4/4)

• Whataboutnow?

• ...Sinceemployeescanhavemultiplehomedepartments,itmustbearelationshipattribute

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Home0..*1..*

homerelationship

ssn name lot code name

Employee Department

startdate

startdate startdate

RelationshipSetRoleNames

• Relationshipscanhaverolenames• Anemployee“manages”zerooronedepartment

• Adepartmentis“managedby”exactlyoneemployee

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Manager0..11..1

ssn name lot code name

Employee Department

Note:somenotationsusetheoppositesideoftherelationshipsettospecifycardinalityandroles

managedby manages

RelationshipSet

• Entitysetscanparticipateindifferentrolesforthesamerelationshipset

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Manager0..*

ssn name lot

Employeesupervises

0..1

Exercise• Formgroupsof2

• DrawanERDforadatabasethatstoresinformationaboutStudents,Faculty,Courses,andCourseOfferings• Facultycanserveasa“coursecoordinator”

• Facultycanbequalifiedtoteachacourse

• Coursescanhaveothercoursesasprerequisites

• Oneormorefacultycanteachcourseofferings

• Identifyentitysets,attributes,andkeys

• Identifyrelationships(androles,ifneeded)

• Definecardinalityconstraints

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WeakEntitySets(1/)

• Inthismodelassume• weneedtorecordtheinsurancepoliciesofemployees

• weneedtotrackdependentsw.r.t.thepolicies

• weonlyneedtostorethenameanddate-of-birthofdependents(andnothingelse)

• that,e.g.,whenemployeesleave,wenolongertracktheirpoliciesordependents

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Policy

ssn name lot name dob

Employee Dependents

WeakEntitySets(2/)

• Notethatweonlyidentifydependentsthroughtheircorrespondingemployees• weassumedependentsofanemployeehaveuniquenames

• butdifferentemployeescouldhavedependentswiththesamename...sincenamesarenotguaranteedtobeunique,e.g.,“John Smith”

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Policy

ssn name lot

Employee Dependents

name dob

WeakEntitySets(3/)

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Policy

ssn name lot

Employee Dependents

Thisis“strong”entityset Thisis“weak”entityset

Thisis“identifying”relationship

(Itwouldnotbeinthedatabaseifthestrongentitywerenotpresent)

name dob

WeakEntitySets(4/)

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Policy

ssn name lot

Employee Dependents

name dob

1..1 0..*

Thisis“strong”entityset Thisis“weak”entityset

Thisis“identifying”relationship

(Itwouldnotbeinthedatabaseifthestrongentitywerenotpresent)

Thisis“partial”entityset(mustbecombinedwiththestrongentities’key,ssn,toidentifythedependencies)

WeakEntitySets(5/)

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Policy

ssn name lot

Employee Dependents

Cardinalitiesforanidentifyingrelationshipset

name dob

1..1 0..*

Theymust belikethis

TernaryversusBinaryRelationships(1/4)

• Thesetwoschemasarenotequivalent!

• Whenwouldweuseaternaryrelationshipset?

• Whenwouldweusethreebinaryrelationshipsets?

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Product Supplier

Project

Supplies

Product Supplier

Project

Provides

AuthorizesRequires

TernaryversusBinaryRelationships (2/4)

• Thisternaryrelationshipsetmeansthatasuppliermustbeauthorizedtosupplyaparticularparttoaparticularproject

• Forexample• “officedepot”cansupply“printerpaper”to“project112”• “officemax”cansupply“paperclips”to“project115”

• butthisdoesnotimply“officemax”cansupply“paperclips”to“112”

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Product Supplier

Project

Supplies

Product Supplier

Project

Provides

AuthorizesRequires

TernaryversusBinaryRelationships (3/4)

• Thisternaryrelationshipsetmeansthatasuppliermustbeauthorizedtosupplyaparticularparttoaparticularproject

• Forexample• “officedepot”cansupply“printerpaper”to“project112”

• “officemax”cansupply“paperclips”to“project115”

• butthisdoesnotimply“officemax”cansupply“paperclips”to“112”

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Product Supplier

Project

Supplies

papers officedepot

project112

suppliespaperclips officemax

project115

supplies

TernaryversusBinaryRelationships (4/4)

• Eachbinaryrelationshipsetrepresentssomethingdistinct• asuppliercanprovidecertainproducts(officemaxcanprovidepencils)

• aprojectcanrequirecertainproducts(112requirespencils)

• asuppliercanbeauthorizedtouseacertainsupplier(112isauthorizedtouseofficemax)

• therefore,wemightassumethatoffice-maxsuppliespencilsto112

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Product Supplier

Project

Provides

AuthorizesRequires

officedepotpaper provides

officemaxclips provides

project112paper requires

project115officemax authorizes

Providing,Authorizing,andRequiringdonotimplysupplying!

Duality:EntityvsValue/AttributevsRelationship(1/3)

• ShouldofficebeanattributeofEmployee?

• ShouldofficebeaseparateEntitySet?• Mostattributescanbe“promoted”toanEntitySet...andsomeEntitiescanbe“demoted”toanattributevalue

• Thisisonereasonwhytherearesomanydifferentwaystodesignaschema

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Assignment

num name due ssn name

Project Employee

office

Duality:EntityvsValue/AttributevsRelationship(2/3)

• WhataresomereasonstomodelOfficeasanEntitySet?• Anemployeecanhavemorethanoneoffice

• ThereareotherattributesofOffice

• Officeneedstoparticipateinotherrelationshipsets

• E.g.,arelationshipsetconnectingtofurnitureortelephonesornetworkdrops(locatedintheoffice)

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Assignment

num name due ssn name

Project Employee Assignedoffice

num

Office

Duality:EntityvsValue/AttributevsRelationship(3/3)

• ExamplewhenOfficeshouldbeanentityset• Officeneedstoparticipateinotherrelationshipsets

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Assignment

num name due ssn name

Project Employee Assignedoffice

num

Office

LocatedinFurniture

type

id

TranslatingERDstoRelationalSchemas

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TranslatingERDstoRelationalSchemas

• Thismappingassumes:• Employeeshaveonehomedepartment

• Departmentshaveonemanager

• Employeescanparticipateinmanyprojects

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Employee Department

Project

Home

Manager

Projectmember

Sponsor

ssn name lot code name

number

name

startdate

enddate

budget

Employee(ssn, name, lot, home-dept) ProjectMember(ssn, number) Department(code, name, manager)Project(number, name, startdate, enddate, budget, sponsor)

TranslatingRelationshipSets(1/7)• Forrelationshipsets• wemustindicatewhichentitieswewanttohaveconnected

• e.g.,weneedthekeyvaluesforemployeesandteamsstoredtogethertorepresenttherelationship

• thesecouldbeinanexistingtablethatrepresentsoneoftheinvolvedentities...

• ...orinanewtableintroducedexplicitlytorepresenttherelationship

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TeamMember

ssn name lot num name

Employee Department

start

TranslatingRelationshipSets(2/7)• Formany-to-manyrelationshipsets• createanewtabletorepresenttherelationship

• Forexample:

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TeamMember

ssn name lot num name

Employee Department

start

TeamMember(ssn,num)...withtwoforeignkeys

Employee(ssn,name,lot)Team(num,name,start)

0..* 0..*

TranslatingRelationshipSets(3/7)• Forone-to-manyrelationshipsets• introduceaforeignkeytothe“many”sideoftherelationship

• Forexample:

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Home

ssn name lot code name

Employee Department

Department(code,name)Employee(ssn,name,lot,homedept)...wherehomedept isaforeignkey(referencingDepartmentcode)

0..* 0..1

TranslatingRelationshipSets(4/7)• Alternatively,forone-to-manyrelationshipsets• Createanewtable(likeinmany-to-manyrelationships)

• Forexample:

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Home

ssn name lot code name

Employee Department

HomeDepartment(ssn,code)Department(code,name)Employee(ssn,name,lot)Whatarethetradeoffsbetweentheseapproaches?...

0..* 0..1

...notethatssn isthekey

TranslatingRelationshipSets(5/7)• Creatinganewtablefromone-to-manyrelationships• requiresaJointoobtainanemployeeshomedepartment

• EachEmployeessn valueassociatedwithahomedepartmentisstoredtwice

• ...intheEmployeeandHomeDepartment table

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Home

ssn name lot code name

Employee Department0..* 0..1

TranslatingRelationshipSets(6/7)• Whenamany-manyrelationshipsethasattributes• Puttheminthetablethatrepresentstherelationship

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ProjectTeam

ssn name lot num name

Employee Project0..* 0..*

ProjectTeam(num,ssn,role,startdate,enddate)Project(num,name,startdate,enddate,budget)Employee(ssn,name,office)

startdate

enddaterole

enddate

startdate

budget

TranslatingRelationshipSets(7/7)• Whenaone-to-manyrelationshipsethasattributes• Addthemtothetablewheretherelationshipisrepresented

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Manager

ssn name lot code name

Employee Department1..* 0..1

Department(code,name,manager,startdate,enddate)Employee(ssn,name,office,dept)

startdate enddate

orelse…

Manager(code,ssn,startdate,enddate)

ParticipationConstraintsinSQL• Wecanrequireanytabletobeinabinaryrelationshipusingaforeignkey• byconstrainingtheattributetobeNOTNULL

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CREATETABLEDepartment(codeINTEGER, nameVARCHAR(20),managerVARCHAR(9)NOTNULL,startdate DATE,enddate DATE,PRIMARYKEY(code),FOREIGNKEY(manager)REFERENCESEmployee,ONDELETENOACTION

);

Manager

ssn name lot code name

Employee Project1..1 0..1

startdate enddate

TranslatingWeakEntitySets• Weakentitysetsandidentifyingrelationshipsetsaretranslatedintoasingletable

• Wemustincludethekeyofthestrongentityasaforeignkey

• Thekeyforthetableisthekeyofthestrongentityplusthepartialkey

• Whentheownerentityisdeleted,allownedweakentitiesmustalsobedeleted

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CREATETABLEPolicy(nameVARCHAR(20),ageINTEGER,ssn VARCHAR(11)NOTNULL,PRIMARYKEY(name,ssn)REFERENCESEmployee,ONDELETECASCADE

);

SummaryoftheTranslation[Elmasri &Navathe]• Createtableandchoosekeyforeachentityset,include(single-valued)attributes

• Createtableforeachweakentityset,include(single-valued)attributesandthekeyoftheownerasaforeignkey.Setthekeyasforeignkeypluspartialkey.

• Foreachone-to-onerelationshipset,addaforeignkeytooneoftheentitysetsinvolvedintherelationship(aforeignkeytotheotherentityintherelationship)*

• Foreachone-to-manyrelationshipset,addaforeignkeytotheentitysetonthemanysideoftherelationship(toreferencetheentitysetontheonesideoftherelationship)*

• Foreachmany-to-manyrelationshipset,createanewtable.Includeaforeignkeyforeachparticipantentitysetintherelationshipset.The keyforthenewtableisthesetofallsuchforeignkeys.

• Foreachmulti-valuedattribute,constructaseparatetable.Repeatthekeyfortheentityinthisnewtable.Itservesasbothakeyforthistableandaforeignkeytotheoriginaltablefortheentity.

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*Unlesstherelationshipsethasattributes,inwhichcasecreateanewtablefortherelationshipset

ForNextWeek• Review– Quizonthematerial• Ch.2-2.5

• Ch.3.5

• Readingassignments• Ch.19to19.6

• Besureyouunderstand

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