数据库系统概念全套配套课件PPT ch2.ppt 30页

'Chapter2:IntrotoRelationalModel ExampleofaRelationattributes(orcolumns)tuples(orrows) AttributeTypesThesetofallowedvaluesforeachattributeiscalledthedomainoftheattributeAttributevaluesare(normally)requiredtobeatomic;thatis,indivisibleThespecialvaluenullisamemberofeverydomainThenullvaluecausescomplicationsinthedefinitionofmanyoperations RelationSchemaandInstanceA1,A2,…,AnareattributesR=(A1,A2,…,An)isarelationschemaExample:instructor=(ID,name,dept_name,salary)Formally,givensetsD1,D2,….DnarelationrisasubsetofD1xD2x…xDnThus,arelationisasetofn-tuples(a1,a2,…,an)whereeachaiDiThecurrentvalues(relationinstance)ofarelationarespecifiedbyatableAnelementtofrisatuple,representedbyarowinatable RelationsareUnorderedOrderoftuplesisirrelevant(tuplesmaybestoredinanarbitraryorder)Example:instructorrelationwithunorderedtuples DatabaseAdatabaseconsistsofmultiplerelationsInformationaboutanenterpriseisbrokenupintopartsinstructorstudentadvisorBaddesign:univ(instructor-ID,name,dept_name,salary,student_Id,..)resultsinrepetitionofinformation(e.g.,twostudentshavethesameinstructor)theneedfornullvalues(e.g.,representanstudentwithnoadvisor)Normalizationtheory(Chapter7)dealswithhowtodesign“good”relationalschemas KeysLetKRKisasuperkeyofRifvaluesforKaresufficienttoidentifyauniquetupleofeachpossiblerelationr(R)Example:{ID}and{ID,name}arebothsuperkeysofinstructor.SuperkeyKisacandidatekeyifKisminimalExample:{ID}isacandidatekeyforInstructorOneofthecandidatekeysisselectedtobetheprimarykey.whichone?Foreignkeyconstraint:ValueinonerelationmustappearinanotherReferencingrelationReferencedrelation SchemaDiagramforUniversityDatabase RelationalQueryLanguagesProceduralvs.non-procedural,ordeclarative“Pure”languages:RelationalalgebraTuplerelationalcalculusDomainrelationalcalculusRelationaloperators SelectionoftuplesRelationrSelecttupleswithA=BandD>5σA=BandD>5(r) SelectionofColumns(Attributes)Relationr:SelectAandCProjectionΠA,C(r) Joiningtworelations–CartesianProductRelationsr,s:rxs: UnionoftworelationsRelationsr,s:rs: SetdifferenceoftworelationsRelationsr,s:r–s: SetIntersectionoftworelationsRelationr,s:rs Joiningtworelations–NaturalJoinLetrandsberelationsonschemasRandSrespectively.Then,the“naturaljoin”ofrelationsRandSisarelationonschemaRSobtainedasfollows:Considereachpairoftuplestrfromrandtsfroms.IftrandtshavethesamevalueoneachoftheattributesinRS,addatuplettotheresult,wherethasthesamevalueastronrthasthesamevalueastsons NaturalJoinExampleRelationsr,s:NaturalJoinrs Figurein-2.1 EndofChapter2 Figure2.01 Figure2.02 Figure2.03 Figure2.04 Figure2.05 Figure2.06 Figure2.07 Figure2.10 Figure2.11 Figure2.12 Figure2.13'