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1、..WORD.格式整理..Exercise3.1.1Answersforthisexercisemayvarybecauseofdifferentinterpretations.SomepossibleFDs:SocialSecuritynumberànameAreacodeàstateStreetaddress,city,stateàzipcodePossiblekeys:{SocialSecuritynumber,streetaddress,city,state,areacode,phonenumber}Needstreetaddress,city,statetouniquelydeter
2、minelocation.Apersoncouldhavemultipleaddresses.Thesameistrueforphones.Thesedays,apersoncouldhavealandlineandacellularphoneExercise3.1.2AnswersforthisexercisemayvarybecauseofdifferentinterpretationsSomepossibleFDs:IDàx-position,y-position,z-positionIDàx-velocity,y-velocity,z-velocityx-position,y-posi
3、tion,z-positionàIDPossiblekeys:{ID}{x-position,y-position,z-position}Thereasonwhythepositionswouldbeakeyisnotwomoleculescanoccupythesamepoint.Exercise3.1.3aThesuperkeysareanysubsetthatcontainsA1.Thus,thereare2(n-1)suchsubsets,sinceeachofthen-1attributesA2throughAnmayindependentlybechoseninorout.Exer
4、cise3.1.3bThesuperkeysareanysubsetthatcontainsA1orA2.Thereare2(n-1)suchsubsetswhenconsideringA1andthen-1attributesA2throughAn.Thereare2(n-2)suchsubsetswhenconsideringA2andthen-2attributesA3throughAn.WedonotcountA1inthesesubsetsbecausetheyarealreadycountedinthefirstgroupofsubsets.Thetotalnumberofsubs
5、etsis2(n-1)+2(n-2).Exercise3.1.3c..专业.知识.分享....WORD.格式整理..Thesuperkeysareanysubsetthatcontains{A1,A2}or{A3,A4}.Thereare2(n-2)suchsubsetswhenconsidering{A1,A2}andthen-2attributesA3throughAn.Thereare2(n-2)–2(n-4)suchsubsetswhenconsidering{A3,A4}andattributesA5throughAnalongwiththeindividualattributesA
6、1andA2.Wegetthe2(n-4)termbecausewehavetodiscardthesubsetsthatcontainthekey{A1,A2}toavoiddoublecounting.Thetotalnumberofsubsetsis2(n-2)+2(n-2)–2(n-4).Exercise3.1.3dThesuperkeysareanysubsetthatcontains{A1,A2}or{A1,A3}.Thereare2(n-2)suchsubsetswhenconsidering{A1,A2}andthen-2attributesA3throughAn.Therea
7、re2(n-3)suchsubsetswhenconsidering{A1,A3}andthen-3attributesA4throughAnWedonotcountA2inthesesubsetsbecausetheyarealreadycountedinthefirstgroupofsubsets.Thetotalnumberofsubsetsis2(n-2)+2(n-3).Exercise3
