数据库系统英第三版课件2012版 Database Systems(3).ppt

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1、3DesignTheoryforRelationalDatabasesFunctionalDependenciesRulesAboutFunctionalDependencies*DesignofRelationalDatabaseSchemasDecomposition:TheGood,Bad,orUglyThirdNormalFormMultivaluedDependenciesAnAlgorithmforDiscoveringMVD’s3.1FunctionalDependencies(函数依赖)I

2、tiscommonforaninitialrelationalschematohaveroomforimprovement,especiallybyeliminatingredundancy(消除冗余).Thereisadesigntheoryforrelationsthatletsusexamineadesigncarefullyandmakeimprovementsbasedonafewsimplepriciples.Ourdiscussionstartswith“functionaldependen

3、cies”,ageneralizationoftheideaofakeyforarelation.3.1.1DefinitionofFunctionalDependencyThefunctionaldependencyXAholdsonRifandonlyifforanylegalrelationsr,wheneveranytwotuplest1andt2ofragreeontheattributesX,theyalsoagreeontheattributeA.Say“Xfunctionallydete

4、rmine(函数决定)A.”Say“Afunctionallydependon(函数依赖于)X.”3.1.1DefinitionofFunctionalDependencyAfunctionaldependency(FD)onarelationRisanassertion:iftwotuplesofRagreeonalloftheattributesA1,A2,…,An,thentheymustalsoagreeonallofanotherlistofattributesB1,B2,…,Bm.Wewrit

5、ethisFDformallyasA1A2…AnB1B2…BmorA={A1,A2,…,An},B={B1,B2,…,Bm}AB3.1.1DefinitionofFunctionalDependencyExample3.1：LetusconsidertherelationMovies(title,year,length,genre,studioName,starName)titleyearlengthgenrestudioNamestarNameStarWarsStarWarsStarWarsGone

6、WiththeWindWayne’sWorldWayne’sWorld1977197719771939199219921241241242319595SciFiSciFiSciFidramacomedycomedyFoxFoxFoxMGMParamountParamountCarrieFisherMarkHamillHarrisonFordVivienLeighDanaCarveyMikeMeyerstitleyearlengthgenrestudioNameconsider：titleyearsta

7、rName?3.1.1DefinitionofFunctionalDependencyRememberthataFD,likeanyconstraint,isanassertionabouttheschemaofarelation,notaboutaparticularinstance.Example:ConsiderR(A,B)withthefollowinginstance.Onthisinstance,ABdoesNOThold,butdoesBAhold?ABa1b1a2b3a1b23.1.1

8、DefinitionofFunctionalDependencyCorollary(推论)1:A1A2…AnB1A1A2…AnB2…A1A2…AnBmiff(ifandonlyif)A1A2…AnB1B2…BmCorollary2:AA3.1.1DefinitionofFunctionalDependencyConsiderthefunctionaldependenciesinrelationstudents(Sno