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ID:21973935
大小:556.50 KB
页数:45页
时间:2018-10-26
《数据库系统基础教程第四章答案解析》由会员上传分享,免费在线阅读,更多相关内容在应用文档-天天文库。
1、SolutionsChapter44.1.14.1.2a)b)c)Incweassumethataphoneandaddresscanonlybelongtoasinglecustomer(1-mrelationshiprepresentedbyarrowintocustomer).d)Indweassumethatanaddresscanonlybelongtoonecustomerandaphonecanexistatonlyoneaddress.Ifthemultiplicityofaboverelationshipswerem-to-n,th
2、eentitysetbecomesweakandthekeyssNoofcustomerswillbeneededaspartofthecompositekeyoftheentityset.Inc&d,weconvertattributesphonesandaddressestoentitysets.Sinceentitysetsoftenbecomerelationsinrelationaldesign,wemustconsidermoreefficientalternatives.Insteadofqueryingmultipletableswh
3、erekeyvaluesareduplicated,wecanalsomodifyattributes:(i)PhonesattributecanbeconvertedintoHomePhone,OfficePhoneandCellPhone.(ii)Amultivaluedattributesuchasaliascanbekeptasanattributewhereasinglecolumncanbeusedinrelationaldesigni.e.concatenateallvalues.SQLallowsaquery"like'%Junius
4、%'"tosearchthemultiplevaluesinacolumnalias.4.1.34.1.4a)b)c)Therelationship"played"betweenTeamsandPlayersissimilartorelationship"plays"betweenTeamsandPlayers.4.1.54.1.6TheinformationaboutchildrencanbeascertainedfrommotherOfandfatherOfrelationships.AttributessNoisrequiredsincenam
5、esarenotunique.4.1.74.1.8a)(b)4.1.9AssumptionsAProfessoronlyworksinatmostonedepartment.AcoursehasatmostoneTA.Acourseisonlytaughtbyoneprofessorandofferedbyonedepartment.Studentsandprofessorshavebeenassigneduniqueemailids.Acourseisuniquelyidentifiedbythecourseno,sectionno,andseme
6、ster(e.g.cs157-3spring09).4.1.10Giventhatforeachmovie,auniquestudioexiststhatproducesthemovie.Eachstariscontractedtoatmostonestudio.Butstarscouldbeunemployedatagiventime.Thusthefour-wayrelationshipinfig4.6canbeeasilyintoconvertedequivalentrelationships.4.2.1Redundancy:Theownera
7、ddressisrepeatedinAccSetsandAddressesentitysets.Simplicity:AccSetsdoesnotserveanyusefulpurposeandthedesigncanbemoresimplyrepresentedbycreatingmany-to-manyrelationshipbetweenCustomersandAccounts.Rightkindofelement:TheentitysetAddresseshasasingleattributeaddress.Acustomercannotha
8、vemorethanoneaddress.Henceaddressshouldbeanattributeof
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