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数据库系统基础教程答案.docx

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Solutions Chapter4 a) b) c) Incweassumethataphoneandaddresscanonlybelongtoasinglecustomer(1-mrelationshiprepresentedbyarrowintocustomer). d) Indweassumethatanaddresscanonlybelongtoonecustomerandaphonecanexistatonlyoneaddress. Ifthemultiplicityofaboverelationshipswerem-to-n,theentitysetbecomesweakandthekeyssNoofcustomerswillbeneededaspartofthecompositekeyoftheentityset. Inc&d,weconvertattributesphonesandaddressestoentitysets.Sinceentitysetsoftenbecomerelationsinrelationaldesign, wemustconsidermoreefficientalternatives. Insteadofqueryingmultipletableswherekeyvaluesareduplicated,wecanalsomodifyattributes: (i)PhonesattributecanbeconvertedintoHomePhone,OfficePhoneandCellPhone. (ii)Amultivaluedattributesuchasaliascanbekeptasanattributewhereasinglecolumncanbeusedinrelationaldesigni.e.concatenateallvalues.SQLallowsaquery"like'%Junius%'"tosearchthemultiplevaluesinacolumnalias. 4.1.3 4.1.4 a) b) c) Therelationship"played"betweenTeamsandPlayersissimilartorelationship"plays"betweenTeamsandPlayers. 4.1.5 4.1.6TheinformationaboutchildrencanbeascertainedfrommotherOfandfatherOfrelationships.AttributessNoisrequiredsincenamesarenotunique. a) (b) Assumptions AProfessoronlyworksinatmostonedepartment. AcoursehasatmostoneTA. Acourseisonlytaughtbyoneprofessorandofferedbyonedepartment. Studentsandprofessorshavebeenassigneduniqueemailids. Acourseisuniquelyidentifiedbythecourseno,sectionno,andsemester(e.g.cs157-3spring09). Giventhatforeachmovie,auniquestudioexiststhatproducesthemovie.Eachstariscontractedtoatmostonestudio. Butstarscouldbeunemployedatagiventime.Thusthefour-wayrelationshipinfig4.6canbeeasilyintoconvertedequivalentrelationships. Redundancy:TheowneraddressisrepeatedinAccSetsandAddressesentitysets. Simplicity:AccSetsdoesnotserveanyusefulpurposeandthedesigncanbemoresimp