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ID:19836346
大小:603.50 KB
页数:45页
时间:2018-10-06
《数据库系统基础教程第四章答案》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,the
2、entitysetbecomesweakandthekeyssNoofcustomerswillbeneededaspartofthecompositekeyoftheentityset.Inc&d,weconvertattributesphonesandaddressestoentitysets.Sinceentitysetsoftenbecomerelationsinrelationaldesign,wemustconsidermoreefficientalternatives.Insteadofqueryingmultipletableswher
3、ekeyvaluesareduplicated,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.AttributessNoisrequiredsincenamesar
5、enotunique.4.1.74.1.8a)(b)4.1.9AssumptionsAProfessoronlyworksinatmostonedepartment.AcoursehasatmostoneTA.Acourseisonlytaughtbyoneprofessorandofferedbyonedepartment.Studentsandprofessorshavebeenassigneduniqueemailids.Acourseisuniquelyidentifiedbythecourseno,sectionno,andsemester(
6、e.g.cs157-3spring09).4.1.10Giventhatforeachmovie,auniquestudioexiststhatproducesthemovie.Eachstariscontractedtoatmostonestudio.Butstarscouldbeunemployedatagiventime.Thusthefour-wayrelationshipinfig4.6canbeeasilyintoconvertedequivalentrelationships.4.2.1Redundancy:Theowneraddress
7、isrepeatedinAccSetsandAddressesentitysets.Simplicity:AccSetsdoesnotserveanyusefulpurposeandthedesigncanbemoresimplyrepresentedbycreatingmany-to-manyrelationshipbetweenCustomersandAccounts.Rightkindofelement:TheentitysetAddresseshasasingleattributeaddress.Acustomercannothavemoret
8、hanoneaddress.Henceaddressshouldbeanattributeof
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