《数据库处理——基础、设计与实现十版》五PPT课件

.,5-1,DavidM.Kroenkes,ChapterFive:DataModelingwiththeEntity-RelationshipModelPartOne,DatabaseProcessing:Fundamentals,Design,andImplementation,.,5-2,TheDataModel,Adatamodelisaplan,orblueprint,foradatabasedesign.Adatamodelismoregeneralizedandabstractthanadatabasedesign.Itiseasiertochangeadatamodelthanitistochangeadatabasedesign,soitistheappropriateplacetoworkthroughconceptualdatabaseproblems.,.,5-3,E-RModel,Entity-Relationshipmodelisasetofconceptsandgraphicalsymbolsthatcanbeusedtocreateconceptualschemas.VersionsOriginalE-RmodelPeterChen(1976).ExtendedE-RmodelExtensionstotheChenmodel.InformationEngineering(IE)JamesMartin(1990);ituses“crowsfoot”notation,iseasiertounderstandandwewilluseit.IDEF1XAnationalstandarddevelopedbytheNationalInstituteofStandardsandTechnologyseeAppendixBUnifiedModelingLanguage(UML)TheObjectManagementGroup;itsupportsobject-orientedmethodologyseeAppendixC,.,5-4,Entities,SomethingthatcanbeidentifiedandtheuserswanttotrackEntityclassacollectionofentitiesofagiventypeEntityinstancetheoccurenceofaparticularentityThereareusuallymanyinstancesofanentityinanentityclass.,.,5-5,CUSTOMER:TheEntityClassandTwoEntityInstances,.,5-6,Attributes,Attributesdescribeanentityscharacteristics.Allentityinstancesofagivenentityclasshavethesameattributes,butvaryinthevaluesofthoseattributes.Originallyshownindatamodelsasellipses.Datamodelingproductstodaycommonlyshowattributesinrectangularform.,.,5-7,EMPLOYEE:AttributesinEllipses,.,5-8,EMPLOYEE:AttributesinEntityRectangle,.,5-9,Identifiers,Identifiersareattributesthatname,oridentify,entityinstances.Theidentifierofanentityinstanceconsistsofoneormoreoftheentitysattributes.Compositeidentifiers:IdentifiersthatconsistoftwoormoreattributesIdentifiersindatamodelsbecomekeysindatabasedesigns:Entitieshaveidentifiers.Tables(orrelations)havekeys.,.,5-10,EntityAttributeDisplayinDataModels,.,5-11,Relationships,Entitiescanbeassociatedwithoneanotherinrelationships:Relationshipclasses:associationsamongentityclassesRelationshipinstances:associationsamongentityinstancesIntheoriginalE-Rmodel,relationshipscouldhaveattributesbuttodaythisisnolongerdone.Arelationshipclasscaninvolvetwoormoreentityclasses.,.,5-12,DegreeoftheRelationship,Thedegreeoftherelationshipisthenumberofentityclassesintherelationship:Twoentitieshaveabinaryrelationshipofdegreetwo.Threeentitieshaveaternaryrelationshipofdegreethree.,.,5-13,BinaryRelationship,.,5-14,TernaryRelationship,.,5-15,EntitiesandTables,Theprincipledifferencebetweenanentityandatable(relation)isthatyoucanexpressarelationshipbetweenentitieswithoutusingforeignkeys.Thismakesiteasiertoworkwithentitiesintheearlydesignprocesswheretheveryexistenceofentitiesandtherelationshipsbetweenthemisuncertain.,.,5-16,Cardinality,Cardinalitymeans“count,”andisexpressedasanumber.Maximumcardinalityisthemaximumnumberofentityinstancesthatcanparticipateinarelationship.Minimumcardinalityistheminimumnumberofentityinstancesthatmustparticipateinarelationship.,.,5-17,MaximumCardinality,Maximumcardinalityisthemaximumnumberofentityinstancesthatcanparticipateinarelationship.Therearethreetypesofmaximumcardinality:One-to-One1:1One-to-Many1:NMany-to-ManyN:M,.,5-18,TheThreeTypesofMaximumCardinality,.,5-19,ParentandChildEntities,Inaone-to-manyrelationship:Theentityontheonesideoftherelationshipiscalledtheparententityorjusttheparent.Theentityonthemanysideoftherelationshipiscalledthechildentityorjustthechild.Inthefigurebelow,EMPLOYEEistheparentandCOMPUTERisthechild:,.,5-20,HAS-ARelationships,TherelationshipswehavebeendiscussingareknownasHAS-Arelationships:Eachentityinstancehasarelationshipwithanotherentityinstance:AnEMPLOYEEhasoneormoreCOMPUTERs.ACOMPUTERhasanassignedEMPLOYEE.,.,5-21,MinimumCardinality,Minimumcardinalityistheminimumnumberofentityinstancesthatmustparticipateinarelationship.Minimumsaregenerallystatedaseitherzeroorone:IFzero0THENparticipationintherelationshipbytheentityisoptional,andnoentityinstancemustparticipateintherelationship.IFone1THENparticipationintherelationshipbytheentityismandatory,andatleastoneentityinstancemustparticipateintherelationship.,.,5-22,IndicatingMinimumCardinality,Asshownintheexamplesinafollowingslide:Minimumcardinalityofzero0indicatingoptionalparticipationisindicatedbyplacinganovalnexttotheoptionalentity.Minimumcardinalityofone1indicatingmandatory(required)participationisindicatedbyplacingaverticalhashmarknexttotherequiredentity.,.,5-23,ReadingMinimumCardinality,Looktowardtheentityinquestion:IFyouseeanovalTHENthatentityisoptional(minimumcardinalityofzero0).IFyouseeaverticalhashmarkTHENthatentityismandatory(required)(minimumcardinalityofone1).,.,5-24,TheThreeTypesofMinimumCardinality,.,5-25,DataModelingNotation,.,5-26,DataModelingNotation:ERwin,.,5-27,DataModelingNotation:N:MandO-M,Notethat:(1)ERwincannotindicatetrueminimumcardinalitiesonN:Mrelationships(2)VisiointroducestheintersectiontableinsteadofusingatrueN:Mmodel,.,5-28,ID-DependentEntities,AnID-dependententityisanentity(child)whoseidentifierincludestheidentifierofanotherentity(parent).TheID-dependententityisalogicalextensionorsub-unitoftheparent:BUILDING:APARTMENTPAINTING:PRINTTheminimumcardinalityfromtheID-dependententitytotheparentisalwaysone.,.,5-29,ID-DependentEntities,Asolidlineindicatesanidentifyingrelationship,.,5-30,WeakEntities,Aweakentityisanentitywhoseexisitencedependsuponanotherentity.AllID-Dependententitiesareconsideredweak.Buttherearealsonon-ID-dependentweakentities.Theidentifieroftheparentdoesnotappearintheidentifieroftheweakchildentity.,.,5-31,WeakEntities(Continued),Adashedlineindicatesanonidentifyingrelationship,WeakentitiesmustbeindicatedbyanaccompanyingtextboxinErwinThereisnospecificnotationforanonidentifyingbutweakentityrelationship,.,5-32,ID-DependentandWeakEntities,.,5-33,SubtypeEntities,Asubtypeentityisaspecialcaseofasupertypeentity:STUDENT:UNDERGRADUATEorGRADUATEThesupertypecontainsallcommonattributes,whilethesubtypescontainspecificattributes.Thesupertypemayhaveadiscriminatorattributethatindicatesthesubtype.,.,5-34,SubtypeswithaDiscriminator,.,5-35,Subtypes:ExclusiveorInclusive,Ifsubtypesar