《数据库系统概念教学》ch课件

Chapter7:Entity-RelationshipModel.Chapter7:Entity-RelationshiChapter7:Entity-RelationshipModelDesignProcessModelingConstraintsE-RDiagramDesignIssuesWeakEntitySetsExtendedE-RFeaturesDesignoftheBankDatabaseReductiontoRelationSchemasDatabaseDesignUMLChapter7:Entity-RelationshiModelingAdatabasecanbemodeledas:acollectionofentities,relationshipamongentities.Anentity

isanobjectthatexistsandisdistinguishablefromotherobjects.Example:specificperson,company,event,plantEntitieshaveattributesExample:peoplehavenamesandaddresses Anentitysetisasetofentitiesofthesametypethatsharethesameproperties.Example:setofallpersons,companies,trees,holidaysModelingAdatabasecanbemodeEntitySetsinstructorandstudentinstructor_IDinstructor_namestudent-IDstudent_nameEntitySetsinstructorandstuRelationshipSetsArelationshipisanassociationamongseveralentities Example:

44553(Peltier)

advisor 22222(Einstein)

studententity relationshipset instructorentityArelationshipsetisamathematicalrelationamongn

2entities,eachtakenfromentitysets {(e1,e2,…en)|e1E1,e2E2,…,enEn}

where(e1,e2,…,en)isarelationshipExample: (44553,22222)advisorRelationshipSetsArelationshiRelationshipSetadvisorRelationshipSetadvisorRelationshipSets(Cont.)Anattributecanalsobepropertyofarelationshipset.Forinstance,theadvisorrelationshipsetbetweenentitysetsinstructorandstudentmayhavetheattributedatewhichtrackswhenthestudentstartedbeingassociatedwiththeadvisorRelationshipSets(Cont.)AnatDegreeofaRelationshipSetbinaryrelationshipinvolvetwoentitysets(ordegreetwo).mostrelationshipsetsinadatabasesystemarebinary.Relationshipsbetweenmorethantwoentitysetsarerare.Mostrelationshipsarebinary.(Moreonthislater.)Example:studentsworkonresearchprojectsundertheguidanceofaninstructor.relationshipproj_guideisaternaryrelationshipbetweeninstructor,student,andprojectDegreeofaRelationshipSetbiAttributesAnentityisrepresentedbyasetofattributes,thatisdescriptivepropertiespossessedbyallmembersofanentityset.Example:

instructor=(ID,name,street,city,salary)

course=(course_id,title,credits)Domain–thesetofpermittedvaluesforeachattributeAttributetypes:Simpleandcompositeattributes.Single-valuedandmultivaluedattributesExample:multivaluedattribute:phone_numbersDerivedattributesCanbecomputedfromotherattributesExample:age,givendate_of_birthAttributesAnentityisrepreseCompositeAttributesCompositeAttributesMappingCardinalityConstraintsExpressthenumberofentitiestowhichanotherentitycanbeassociatedviaarelationshipset.Mostusefulindescribingbinaryrelationshipsets.Forabinaryrelationshipsetthemappingcardinalitymustbeoneofthefollowingtypes:OnetooneOnetomanyManytooneManytomanyMappingCardinalityConstraintMappingCardinalitiesOnetooneOnetomanyNote:SomeelementsinAandBmaynotbemappedtoanyelementsintheothersetMappingCardinalitiesOnetoonMappingCardinalitiesManytooneManytomanyNote:SomeelementsinAandBmaynotbemappedtoanyelementsintheothersetMappingCardinalitiesManytoKeysAsuperkeyofanentitysetisasetofoneormoreattributeswhosevaluesuniquelydetermineeachentity.AcandidatekeyofanentitysetisaminimalsuperkeyIDiscandidatekeyofinstructorcourse_idiscandidatekeyofcourseAlthoughseveralcandidatekeysmayexist,oneofthecandidatekeysisselectedtobetheprimarykey.KeysAsuperkeyofanentitysKeysforRelationshipSetsThecombinationofprimarykeysoftheparticipatingentitysetsformsasuperkeyofarelationshipset.(s_id,i_id)isthesuperkeyofadvisorNOTE:thismeansapairofentitysetscanhaveatmostonerelationshipinaparticularrelationshipset.

Example:ifwewishtotrackmultiplemeetingdatesbetweenastudentandheradvisor,wecannotassumearelationshipforeachmeeting.WecanuseamultivaluedattributethoughMustconsiderthemappingcardinalityoftherelationshipsetwhendecidingwhatarethecandidatekeysNeedtoconsidersemanticsofrelationshipsetinselectingtheprimarykeyincaseofmorethanonecandidatekeyKeysforRelationshipSetsTheRedundantAttributesSupposewehaveentitysetsinstructor,withattributesincludingdept_namedepartmentandarelationshipinst_deptrelatinginstructoranddepartmentAttributedept_nameinentityinstructorisredundantsincethereisanexplicitrelationshipinst_deptwhichrelatesinstructorstodepartmentsTheattributereplicatesinformationpresentintherelationship,andshouldberemovedfrominstructorBUT:whenconvertingbacktotables,insomecasestheattributegetsreintroduced,aswewillsee.RedundantAttributesSupposeweE-RDiagramsRectanglesrepresententitysets.Diamondsrepresentrelationshipsets.AttributeslistedinsideentityrectangleUnderlineindicatesprimarykeyattributesE-RDiagramsRectanglesrepreseEntityWithComposite,Multivalued,andDerivedAttributesEntityWithComposite,MultivaRelationshipSetswithAttributesRelationshipSetswithAttribuRolesEntitysetsofarelationshipneednotbedistinctEachoccurrenceofanentitysetplaysa“role”intherelationshipThelabels“course_id”and“prereq_id”arecalledroles.RolesEntitysetsofarelationCardinalityConstraintsWeexpresscardinalityconstraintsbydrawingeitheradirectedline(),signifying“one,”oranundirectedline(—),signifying“many,”betweentherelationshipsetandtheentityset.One-to-onerelationship:AstudentisassociatedwithatmostoneinstructorviatherelationshipadvisorAstudentisassociatedwithatmostonedepartmentviastud_deptCardinalityConstraintsWeexprOne-to-OneRelationshipone-to-onerelationshipbetweenaninstructorandastudentaninstructorisassociatedwithatmostonestudentviaadvisor

andastudentisassociatedwithatmostoneinstructorviaadvisorOne-to-OneRelationshipone-to-One-to-ManyRelationshipone-to-manyrelationshipbetweenaninstructorandastudentaninstructorisassociatedwithseveral(including0)studentsviaadvisorastudentisassociatedwithatmostoneinstructorviaadvisor,One-to-ManyRelationshipone-toMany-to-OneRelationshipsInamany-to-onerelationshipbetweenaninstructorandastudent,aninstructor

isassociatedwithatmostonestudentviaadvisor,andastudentisassociatedwithseveral(including0)instructorsviaadvisorMany-to-OneRelationshipsInaMany-to-ManyRelationshipAninstructorisassociatedwithseveral(possibly0)studentsviaadvisorAstudentisassociatedwithseveral(possibly0)instructorsviaadvisor

Many-to-ManyRelationshipAninParticipationofanEntitySetinaRelationshipSetTotalparticipation(indicatedbydoubleline):everyentityintheentitysetparticipatesinatleastonerelationshipintherelationshipsetE.g.,participationofsectioninsec_courseistotaleverysectionmusthaveanassociatedcoursePartialparticipation:someentitiesmaynotparticipateinanyrelationshipintherelationshipsetExample:participationofinstructorinadvisorispartialParticipationofanEntitySetAlternativeNotationforCardinalityLimitsCardinalitylimitscanalsoexpressparticipationconstraintsAlternativeNotationforCardiE-RDiagramwithaTernaryRelationshipE-RDiagramwithaTernaryRelCardinalityConstraintsonTernaryRelationshipWeallowatmostonearrowoutofaternary(orgreaterdegree)relationshiptoindicateacardinalityconstraintE.g.,anarrowfromproj_guidetoinstructorindicateseachstudenthasatmostoneguideforaprojectIfthereismorethanonearrow,therearetwowaysofdefiningthemeaning.E.g.,aternaryrelationshipRbetweenA,BandCwitharrowstoBandCcouldmean1.eachAentityisassociatedwithauniqueentityfromBandCor 2.eachpairofentitiesfrom(A,B)isassociatedwithauniqueCentity,andeachpair(A,C)isassociatedwithauniqueBEachalternativehasbeenusedindifferentformalismsToavoidconfusionweoutlawmorethanonearrowCardinalityConstraintsonTerHowaboutdoinganERdesigninteractivelyontheboard?

Suggestanapplicationtobemodeled..HowaboutdoinganERdesigniWeakEntitySetsAnentitysetthatdoesnothaveaprimarykeyisreferredtoasaweakentityset.TheexistenceofaweakentitysetdependsontheexistenceofaidentifyingentitysetItmustrelatetotheidentifyingentitysetviaatotal,one-to-manyrelationshipsetfromtheidentifyingtotheweakentitysetIdentifyingrelationshipdepictedusingadoublediamondThediscriminator

(orpartialkey)ofaweakentitysetisthesetofattributesthatdistinguishesamongalltheentitiesofaweakentityset.Theprimarykeyofaweakentitysetisformedbytheprimarykeyofthestrongentitysetonwhichtheweakentitysetisexistencedependent,plustheweakentityset’sdiscriminator.WeakEntitySetsAnentitysetWeakEntitySets(Cont.)Weunderlinethediscriminatorofaweakentitysetwithadashedline.Weputtheidentifyingrelationshipofaweakentityinadoublediamond.Primarykeyforsection–(course_id,sec_id,semester,year)WeakEntitySets(Cont.)WeundWeakEntitySets(Cont.)Note:theprimarykeyofthestrongentitysetisnotexplicitlystoredwiththeweakentityset,sinceitisimplicitintheidentifyingrelationship.Ifcourse_idwereexplicitlystored,sectioncouldbemadeastrongentity,butthentherelationshipbetweensectionandcoursewouldbeduplicatedbyanimplicitrelationshipdefinedbytheattributecourse_idcommontocourseandsectionWeakEntitySets(Cont.)Note:E-RDiagramforaUniversityEnterpriseE-RDiagramforaUniversityEReductiontoRelationalSchemasReductiontoRelationalSchemaReductiontoRelationSchemasEntitysetsandrelationshipsetscanbeexpresseduniformlyasrelationschemasthatrepresentthecontentsofthedatabase.AdatabasewhichconformstoanE-Rdiagramcanberepresentedbyacollectionofschemas.Foreachentitysetandrelationshipsetthereisauniqueschemathatisassignedthenameofthecorrespondingentitysetorrelationshipset.Eachschemahasanumberofcolumns(generallycorrespondingtoattributes),whichhaveuniquenames.ReductiontoRelationSchemasERepresentingEntitySetsWithSimpleAttributesAstrongentitysetreducestoaschemawiththesameattributes

student(ID,name,tot_cred)Aweakentitysetbecomesatablethatincludesacolumnfortheprimarykeyoftheidentifyingstrongentityset

section(course_id,sec_id,sem,year)RepresentingEntitySetsWithRepresentingRelationshipSetsAmany-to-manyrelationshipsetisrepresentedasaschemawithattributesfortheprimarykeysofthetwoparticipatingentitysets,andanydescriptiveattributesoftherelationshipset.Example:schemaforrelationshipsetadvisor

advisor=(s_id,i_id)RepresentingRelationshipSetsRedundancyofSchemasMany-to-oneandone-to-manyrelationshipsetsthataretotalonthemany-sidecanberepresentedbyaddinganextraattributetothe“many”side,containingtheprimarykeyofthe“one”sideExample:Insteadofcreatingaschemaforrelationshipsetinst_dept,addanattributedept_nametotheschemaarisingfromentitysetinstructorRedundancyofSchemasMany-to-oRedundancyofSchemas(Cont.)Forone-to-onerelationshipsets,eithersidecanbechosentoactasthe“many”sideThatis,extraattributecanbeaddedtoeitherofthetablescorrespondingtothetwoentitysetsIfparticipationispartialonthe“many”side,replacingaschemabyanextraattributeintheschemacorrespondingtothe“many”sidecouldresultinnullvaluesTheschemacorrespondingtoarelationshipsetlinkingaweakentitysettoitsidentifyingstrongentitysetisredundant.Example:Thesectionschemaalreadycontainstheattributesthatwouldappearinthesec_courseschemaRedundancyofSchemas(Cont.)FCompositeandMultivaluedAttributesCompositeattributesareflattenedoutbycreatingaseparateattributeforeachcomponentattributeExample:givenentitysetinstructorwithcompositeattributenamewithcomponentattributesfirst_nameandlast_nametheschemacorrespondingtotheentitysethastwoattributesname_first_nameandname_last_namePrefixomittedifthereisnoambiguityIgnoringmultivaluedattributes,extendedinstructorschemaisinstructor(ID,

first_name,middle_initial,last_name,

street_number,street_name,

apt_number,city,state,zip_code,

date_of_birth)CompositeandMultivaluedAttrCompositeandMultivaluedAttributesAmultivaluedattributeMofanentityEisrepresentedbyaseparateschemaEMSchemaEMhasattributescorrespondingtotheprimarykeyofEandanattributecorrespondingtomultivaluedattributeMExample:Multivaluedattributephone_numberofinstructorisrepresentedbyaschema:

inst_phone=(

ID,phone_number)

EachvalueofthemultivaluedattributemapstoaseparatetupleoftherelationonschemaEMForexample,aninstructorentitywithprimarykey22222andphonenumbers456-7890and123-4567mapstotwotuples:

(22222,456-7890)and(22222,123-4567)

CompositeandMultivaluedAttrMultivaluedAttributes(Cont.)Specialcase:entitytime_slothasonlyoneattributeotherthantheprimary-keyattribute,andthatattributeismultivaluedOptimization:Don’tcreatetherelationcorrespondingtotheentity,justcreatetheonecorrespondingtothemultivaluedattributetime_slot(time_slot_id,day,start_time,end_time)Caveat:time_slotattributeofsection(fromsec_time_slot)cannotbeaforeignkeyduetothisoptimizationMultivaluedAttributes(Cont.)DesignIssuesUseofentitysetsvs.attributes

Useofphoneasanentityallowsextrainformationaboutphonenumbers(plusmultiplephonenumbers)DesignIssuesUseofentitysetDesignIssuesUseofentitysetsvs.relationshipsets

PossibleguidelineistodesignatearelationshipsettodescribeanactionthatoccursbetweenentitiesDesignIssuesUseofentitysetDesignIssuesBinaryversusn-aryrelationshipsets

Althoughitispossibletoreplaceanynonbinary(n-ary,forn>2)relationshipsetbyanumberofdistinctbinaryrelationshipsets,an-aryrelationshipsetshowsmoreclearlythatseveralentitiesparticipateinasinglerelationship.Placementofrelationshipattributes

e.g.,attributedateasattributeofadvisororasattributeofstudentDesignIssuesBinaryversusn-aBinaryVs.Non-BinaryRelationshipsSomerelationshipsthatappeartobenon-binarymaybebetterrepresentedusingbinaryrelationshipsE.g.,Aternaryrelationshipparents,relatingachildtohis/herfatherandmother,isbestreplacedbytwobinaryrelationships,fatherandmotherUsingtwobinaryrelationshipsallowspartialinformation(e.g.,onlymotherbeingknow)Buttherearesomerelationshipsthatarenaturallynon-binaryExample:proj_guideBinaryVs.Non-BinaryRelationConvertingNon-BinaryRelationshipstoBinaryFormIngeneral,anynon-binaryrelationshipcanberepresentedusingbinaryrelationshipsbycreatinganartificialentityset.ReplaceRbetweenentitysetsA,BandC

byanentitysetE,andthreerelationshipsets: 1.RA,relatingEandA2.RB,relatingEandB

3.RC,relatingEandCCreateaspecialidentifyingattributeforEAddanyattributesofRtoEForeachrelationship(ai,bi,ci)inR,create 1.anewentityei

intheentitysetE2.add(ei,ai)toRA 3.add(ei,bi

)toRB

4.add(ei,ci)toRCConvertingNon-BinaryRelationConvertingNon-BinaryRelationships(Cont.)AlsoneedtotranslateconstraintsTranslatingallconstraintsmaynotbepossibleTheremaybeinstancesinthetranslatedschemathat

cannotcorrespondtoanyinstanceofRExercise:addconstraintstotherelationshipsRA,RBandRCtoensurethatanewlycreatedentitycorrespondstoexactlyoneentityineachofentitysetsA,BandCWecanavoidcreatinganidentifyingattributebymakingEaweakentityset(describedshortly)identifiedbythethreerelationshipsetsConvertingNon-BinaryRelationExtendedERFeaturesExtendedERFeaturesExtendedE-RFeatures:SpecializationTop-downdesignprocess;wedesignatesubgroupingswithinanentitysetthataredistinctivefromotherentitiesintheset.Thesesubgroupingsbecomelower-levelentitysetsthathaveattributesorparticipateinrelationshipsthatdonotapplytothehigher-levelentityset.DepictedbyatrianglecomponentlabeledISA(E.g.,instructor“isa”person).Attributeinheritance–alower-levelentitysetinheritsalltheattributesandrelationshipparticipationofthehigher-levelentitysettowhichitislinked.ExtendedE-RFeatures:SpecialSpecializationExampleSpecializationExampleExtendedERFeatures:GeneralizationAbottom-updesignprocess–combineanumberofentitysetsthatsharethesamefeaturesintoahigher-levelentityset.Specializationandgeneralizationaresimpleinversionsofeachother;theyarerepresentedinanE-Rdiagraminthesameway.Thetermsspecializationandgeneralizationareusedinterchangeably.ExtendedERFeatures:GeneraliSpecializationandGeneralization(Cont.)Canhavemultiplespecializationsofanentitysetbasedondifferentfeatures.E.g.,permanent_employeevs.temporary_employee,inadditiontoinstructorvs.secretaryEachparticularemployeewouldbeamemberofoneofpermanent_employeeortemporary_employee,andalsoamemberofoneofinstructor,secretaryTheISArelationshipalsoreferredtoassuperclass-subclass

relationshipSpecializationandGeneralizatDesignConstraintsonaSpecialization/GeneralizationConstraintonwhichentitiescanbemembersofagivenlower-levelentityset.condition-definedExample:allcustomersover65yearsaremembersofsenior-citizenentityset;senior-citizenISAperson.user-definedConstraintonwhetherornotentitiesmaybelongtomorethanonelower-levelentitysetwithinasinglegeneralization.Disjointanentitycanbelongtoonlyonelower-levelentitysetNotedinE-Rdiagrambyhavingmultiplelower-levelentitysetslinktothesametriangleOverlappinganentitycanbelongtomorethanonelower-levelentitysetDesignConstraintsonaSpeciaDesignConstraintsonaSpecialization/Generalization(Cont.)Completenessconstraint--specifieswhetherornotanentityinthehigher-levelentitysetmustbelongtoatleastoneofthelower-levelentitysetswithinageneralization.total:anentitymustbelongtooneofthelower-levelentitysetspartial:anentityneednotbelongtooneofthelower-levelentitysetsDesignConstraintsonaSpeciaAggregationConsidertheternaryrelationshipproj_guide,whichwesawearlierSupposewewanttorecordevaluationsofastudentbyaguideonaprojectAggregationConsidertheternaAggregation(Cont.)Relationshipsetseval_forandproj_guiderepresentoverlappinginformationEveryeval_forrelationshipcorrespondstoaproj_guiderelationshipHowever,someproj_guiderelationshipsmaynotcorrespondtoanyeval_forrelationshipsSowecan’tdiscardtheproj_guiderelationshipEliminatethisredundancyviaaggregationTreatrelationshipasanabstractentityAllowsrelationshipsbetweenrelationshipsAbstractionofrelationshipintonewentityAggregation(Cont.)RelationshiAggregation(Cont.)Withoutintroducingredundancy,thefollowingdiagramrepresents:AstudentisguidedbyaparticularinstructoronaparticularprojectAstudent,instructor,projectcombinationmayhaveanassociatedevaluationAggregation(Cont.)WithoutintRepresentingSpecializationviaSchemasMethod1:Formaschemaforthehigher-levelentityFormaschemaforeachlower-levelentityset,includeprimarykeyofhigher-levelentitysetandlocalattributes

schema

attributes

person ID,name,street,city

student ID,tot_cred

employee ID,salaryDrawback:gettinginformationabout,anemployeerequiresaccessingtworelations,theonecorrespondingtothelow-levelschemaandtheonecorrespondingtothehigh-levelschemaRepresentingSpecializationviRepresentingSpecializationasSchemas(Cont.)Method2:Formaschemaforeachentitysetwithalllocalandinheritedattributes

schema

attributes

person ID,name,street,city

student ID,name,street,city,tot_cred

employee ID,name,street,city,salaryIfspecializationistotal,theschemaforthegeneralizedentityset(person)notrequiredtostoreinformationCanbedefinedasa“view”relationcontainingunionofspecializationrelationsButexplicitschemamaystillbeneededforforeignkeyconstraintsDrawback:name,streetandcitymaybestoredredundantlyforpeoplewhoarebothstudentsandemployeesRepresentingSpecializationasSchemasCorrespondingtoAggregationTorepresentaggregation,createaschemacontainingprimarykeyoftheaggregatedrelationship,theprimarykeyoftheassociatedentitysetanydescriptiveattributesSchemasCorrespondingtoAggreSchemasCorrespondingtoAggregation(Cont.)Forexample,torepresentaggregationmanagesbetweenrelationshipworks_onandentitysetmanager,createaschema

eval_for(s_ID,project_id,i_ID,evaluation_id)Schemaproj_guideisredundantprovidedwearewillingtostorenullvaluesforattributemanager_nameinrelationonschemamanagesSchemasCorrespondingtoAggreE-RDesignDecisionsTheuseofanattributeorentitysettorepresentanobject.Whetherareal-worldconceptisbestexpressedbyanentitysetorarelationshipset.Theuseofaternaryrelationshipversusapairofbinaryrelationships.Theuseofastrongorweakentityset.Theuseofspecialization/generalization–contributestomodularityinthedesign.Theuseofaggregation–cantreattheaggregateentitysetasasingleunitwithoutconcernforthedetailsofitsinternalstructure.E-RDesignDecisionsTheuseofHowaboutdoinganotherERdesigninteractivelyontheboard?.HowaboutdoinganotherERdesSummaryofSymbolsUsedinE-RNotationSummaryofSymbolsUsedinE-RSymbolsUsedinE-RNotation(Cont.)SymbolsUsedinE-RNotation(AlternativeERNotations

Chen,IDE1FX,…AlternativeERNotationsChen,AlternativeERNotationsChenIDE1FX(Crowsfeetnotation)AlternativeERNotationsUML UML:UnifiedModelingLanguageUMLhasmanycomponentstographicallymodeldifferentaspectsofanentiresoftwaresystemUMLClassDiagramscorrespondtoE-RDiagram,butseveraldifferences.UML UML:UnifiedModelingLangERvs.UMLClassDiagrams*NotereversalofpositionincardinalityconstraintdepictionERvs.UMLClassDiagrams*NoteERvs.UMLClassDiagramsERDiagramNotationEquivalentinUML*Generalizationcanusemergedorseparatearrowsindependentofdisjoint/overlappingERvs.UMLClassDiagramsERDiUMLClassDiagrams(Cont.)BinaryrelationshipsetsarerepresentedinUMLbyjustdrawingalineconnectingtheentitysets.Therelationshipsetnameiswrittenadjacenttotheline.Theroleplayedbyanentitysetinarelationshipsetmayalsobespecifiedbywritingtherolenameontheline,adjacenttotheentityset.Therelationshipsetnamemayalternativelybewritteninabox,alongwithattributesoftherelationshipset,andtheboxisconnected,usingadottedline,tothelinedepictingtherelationshipset.UMLClassDiagrams(Cont.)BinaEndofChapter7

.EndofChapter7

.Figure7.01Figure7.01Figure7.02Figure7.02Figure7.03Figure7.03Figure7.04Figure7.04Figure7.05Figure7.05Figure7.06Figure7.06Figure7.07Figure7.07Figure7.08Figure7.08Figure7.09Figure7.09Figure7.10Figure7.10Figure7.11Figure7.11Figure7.12Figure7.12Figure7.13Figure7.13Figure7.14Figure7.14Figure7.15Figure7.15Figure7.17Figure7.17Figure7.18Figure7.18Figure7.19Figure7.19Figure7.20Figure7.20Figure7.21Figure7.21Figure7.22Figure7.22Figure7.23Figure7.23Figure7.24Figure7.24Figure7.25Figure7.25Figure7.26Figure7.26Figure7.27Figure7.27Figure7.28Figure7.28Figure7.29Figure7.29Chapter7:Entity-RelationshipModel.Chapter7:Entity-RelationshiChapter7:Entity-RelationshipModelDesignProcessModelingConstraintsE-RDiagramDesignIssuesWeakEntitySetsExtendedE-RFeaturesDesignoftheBankDatabaseReductiontoRelationSchemasDatabaseDesignUMLChapter7:Entity-RelationshiModelingAdatabasecanbemodeledas:acollectionofentities,relationshipamongentities.Anentity

isanobjectthatexistsandisdistinguishablefromotherobjects.Example:specificperson,company,event,plantEntitieshaveattributesExample:peoplehavenamesandaddresses Anentitysetisasetofentitiesofthesametypethatsharethesameproperties.Example:setofallpersons,companies,trees,holidaysModelingAdatabasecanbemodeEntitySetsinstructorandstudentinstructor_IDinstructor_namestudent-IDstudent_nameEntitySetsinstructorandstuRelationshipSetsArelationshipisanassociationamongseveralentities Example:

44553(Peltier)

advisor 22222(Einstein)

studententity relationshipset instructorentityArelationshipsetisamathematicalrelationamongn

2entities,eachtakenfromentitysets {(e1,e2,…en)|e1E1,e2E2,…,enEn}

where(e1,e2,…,en)isarelationshipExample: (44553,22222)advisorRelationshipSetsArelationshiRelations