Database Management Systemopen.gnu.ac.kr/lecslides/2017-2-DB/DBLec03_Design.pdf · Database Management System Lecture 3 ... •that, e.g., when employees leave, we no longer track

DatabaseManagementSystem

Lecture3

DatabaseDesign– ERModelandERmodeltoRelationalSchemas

*SomematerialsadaptedfromR.Ramakrishnan,J.Gehrke andShawnBowers

Today’sAgenda• ERModel

• ERModeltoRelational

2DatabaseManagementSystem

ERModel

3DatabaseManagementSystem

ConceptualDataModeling• Similartosoftwaredesign...• requirementsgatheringandanalysis

• applicationarchitectureanddesign

• implementationandtesting

• maintenance

• databasedesigninvolvesmultiplesteps(esp.priortocreatingtabledefinitions)

• HerewefocusonconceptualdesignusingtheEntity- RelationshipModel• similartotheuseofUMLdiagramsforsoftwaredesign

DatabaseManagementSystem 4

Entity-Relationship(ER)Diagram[Chen71]

DatabaseManagementSystem 5

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”

DatabaseManagementSystem 6

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

• justasentitiesareinstancesofentitysets,relationshipsare

• instancesofrelationshipsets...

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

• definedbytheparticipatingentitytypesandotherconstraints

• ofteninformallycalleda“relationship”ifeveryoneknowswearetalkingabouttheschema

• ARelationshipSetisalsobecalleda“RelationshipType”

DatabaseManagementSystem 7

Entity-RelationshipModelvs.RelationalModel• Adifferentdatamodelthantherelationalmodel• differentconstructsformodelingschemasanddata

• DBMSsystems(invariousforms)haveevenbeenbuiltontheERmodel...thoughprimarilyusedasadesigntool

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

• TheERmodelhas:• EntitiesandEntitySetswithattributesandentityidentifiers(likekeys)

• RelationshipsandRelationshipSetswithcardinalityconstraints,roles,attributes,etc.

DatabaseManagementSystem 8

MappingERmodelstoRelationalSchemas

• Thismappingassumes:• Employeeshaveonehomedepartment

• Departmentshaveonemanager

• Employeescanparticipateinmanyprojects

DatabaseManagementSystem 9

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

ERCardinalityConstraints

DatabaseManagementSystem 10

AnEmployeecanhave 0or1homeDepartment

Employee Department

Project

Home

Manager

Projectmember

Sponsor

ssn name lot code name

number

name

startdate

enddate

budget

0..1

ERCardinalityConstraints

DatabaseManagementSystem 11

ADepartemnt canhave 0ormanyhomeEmployees

Employee Department

Project

Home

Manager

Projectmember

Sponsor

ssn name lot code name

number

name

startdate

enddate

budget

0..10..*

ERCardinalityConstraints

DatabaseManagementSystem 12

ADepartmentmusthaveexactlyoneManager

Employee Department

Project

Home

Manager

Projectmember

Sponsor

ssn name lot code name

number

name

startdate

enddate

budget

0..10..*

1..1

ERCardinalityConstraints

DatabaseManagementSystem 13

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?

DatabaseManagementSystem 14

Employee DepartmentHome0..11..*

Employee DepartmentHome0..10..*

Employee DepartmentHome1..*0..*

Mustdiscoverthesemanticsoftheapplications

ERCardinalityConstraints(1/5)• ConstraintsareexpressedoverEntitySetsandRelationshipSets

• ConstraintsonthemembersofthecorrespondingEntitiesandRelationships

DatabaseManagementSystem 15

Employee DepartmentHome0..11..*

homerelationship

Doesthissatisfythecardinalityconstraints?

ERCardinalityConstraints (2/5)• ConstraintsareexpressedoverEntitySetsandRelationshipSets

• ConstraintsonthemembersofthecorrespondingEntitiesandRelationships

DatabaseManagementSystem 16

Employee DepartmentHome0..11..*

homerelationship

Howaboutnow?

ERCardinalityConstraints (3/5)• ConstraintsareexpressedoverEntitySetsandRelationshipSets

• ConstraintsonthemembersofthecorrespondingEntitiesandRelationships

DatabaseManagementSystem 17

Employee DepartmentHome0..11..*

homerelationship

Andnow?

ERCardinalityConstraints(4/5)• Therearevariousnotationsusedforwritingcardinalityconstraints...

DatabaseManagementSystem 18

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...

DatabaseManagementSystem 19

Examplesof“ManytoMany”constraints

m n

many many

* *

Maximumcardinalitiesonly

1:n 1:n

one..many one..many

Maximumandmaximumcardinalities

1+ 1+

RelationshipSetAttributes(1/4)

• Eachinstanceoftherelationshiphasavaluefortheattribute...

DatabaseManagementSystem 20

Home0..11..*

homerelationship

ssn name lot code name

Employee Department

startdate

RelationshipSetAttributes (2/4)

• Whichonemakessence?

DatabaseManagementSystem 21

Home0..11..*

homerelationship

ssn name lot code name

Employee Department

startdate

startdate startdate

RelationshipSetAttributes (3/4)

• BecauseEmployeeshavezerooronehomedepartments,

• ...startdatewillworkasanEmpoyee orhomeattribute

• Whataboutstartdate atDepartment?

DatabaseManagementSystem 22

Home0..11..*

homerelationship

ssn name lot code name

Employee Department

startdate

startdate startdate

RelationshipSetAttributes (4/4)

• Whataboutnow?

• ...Sinceemployeescanhavemultiplehomedepartments,itmustbearelationshipattribute

DatabaseManagementSystem 23

Home0..*1..*

homerelationship

ssn name lot code name

Employee Department

startdate

startdate startdate

RelationshipSetRoleNames

• Relationshipscanhaverolenames• Anemployee“manages”zerooronedepartment

• Adepartmentis“managedby”exactlyoneemployee

DatabaseManagementSystem 24

Manager0..11..1

ssn name lot code name

Employee Department

Note:somenotationsusetheoppositesideoftherelationshipsettospecifycardinalityandroles

managedby manages

RelationshipSet

• Entitysetscanparticipateindifferentrolesforthesamerelationshipset

DatabaseManagementSystem 25

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

DatabaseManagementSystem 26

WeakEntitySets(1/)

• Inthismodelassume• weneedtorecordtheinsurancepoliciesofemployees

• weneedtotrackdependentsw.r.t.thepolicies

• weonlyneedtostorethenameanddate-of-birthofdependents(andnothingelse)

• that,e.g.,whenemployeesleave,wenolongertracktheirpoliciesordependents

DatabaseManagementSystem 27

Policy

ssn name lot name dob

Employee Dependents

WeakEntitySets(2/)

• Notethatweonlyidentifydependentsthroughtheircorrespondingemployees• weassumedependentsofanemployeehaveuniquenames

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

DatabaseManagementSystem 28

Policy

ssn name lot

Employee Dependents

name dob

WeakEntitySets(3/)

DatabaseManagementSystem 29

Policy

ssn name lot

Employee Dependents

Thisis“strong”entityset Thisis“weak”entityset

Thisis“identifying”relationship

(Itwouldnotbeinthedatabaseifthestrongentitywerenotpresent)

name dob

WeakEntitySets(4/)

DatabaseManagementSystem 30

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/)

DatabaseManagementSystem 31

Policy

ssn name lot

Employee Dependents

Cardinalitiesforanidentifyingrelationshipset

name dob

1..1 0..*

Theymust belikethis

TernaryversusBinaryRelationships(1/4)

• Thesetwoschemasarenotequivalent!

• Whenwouldweuseaternaryrelationshipset?

• Whenwouldweusethreebinaryrelationshipsets?

DatabaseManagementSystem 32

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”

DatabaseManagementSystem 33

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”

DatabaseManagementSystem 34

Product Supplier

Project

Supplies

papers officedepot

project112

suppliespaperclips officemax

project115

supplies

TernaryversusBinaryRelationships (4/4)

• Eachbinaryrelationshipsetrepresentssomethingdistinct• asuppliercanprovidecertainproducts(officemaxcanprovidepencils)

• aprojectcanrequirecertainproducts(112requirespencils)

• asuppliercanbeauthorizedtouseacertainsupplier(112isauthorizedtouseofficemax)

• therefore,wemightassumethatoffice-maxsuppliespencilsto112

DatabaseManagementSystem 35

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

DatabaseManagementSystem 36

Assignment

num name due ssn name

Project Employee

office

Duality:EntityvsValue/AttributevsRelationship(2/3)

• WhataresomereasonstomodelOfficeasanEntitySet?• Anemployeecanhavemorethanoneoffice

• ThereareotherattributesofOffice

• Officeneedstoparticipateinotherrelationshipsets

• E.g.,arelationshipsetconnectingtofurnitureortelephonesornetworkdrops(locatedintheoffice)

DatabaseManagementSystem 37

Assignment

num name due ssn name

Project Employee Assignedoffice

num

Office

Duality:EntityvsValue/AttributevsRelationship(3/3)

• ExamplewhenOfficeshouldbeanentityset• Officeneedstoparticipateinotherrelationshipsets

DatabaseManagementSystem 38

Assignment

num name due ssn name

Project Employee Assignedoffice

num

Office

LocatedinFurniture

type

id

TranslatingERDstoRelationalSchemas

DatabaseManagementSystem 39

TranslatingERDstoRelationalSchemas

• Thismappingassumes:• Employeeshaveonehomedepartment

• Departmentshaveonemanager

• Employeescanparticipateinmanyprojects

DatabaseManagementSystem 40

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

DatabaseManagementSystem 41

TeamMember

ssn name lot num name

Employee Department

start

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

• Forexample:

DatabaseManagementSystem 42

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:

DatabaseManagementSystem 43

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:

DatabaseManagementSystem 44

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

DatabaseManagementSystem 45

Home

ssn name lot code name

Employee Department0..* 0..1

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

DatabaseManagementSystem 46

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

DatabaseManagementSystem 47

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

DatabaseManagementSystem 48

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

DatabaseManagementSystem 49

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.

DatabaseManagementSystem 50

*Unlesstherelationshipsethasattributes,inwhichcasecreateanewtablefortherelationshipset

ForNextWeek• Review– Quizonthematerial• Ch.2-2.5

• Ch.3.5

• Readingassignments• Ch.19to19.6

• Besureyouunderstand

51DatabaseManagementSystem