数据库系统概念全套配套课件PPT ch6.ppt

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1、Chapter6:FormalRelationalQueryLanguagesChapter6:FormalRelationalQueryLanguagesRelationalAlgebraTupleRelationalCalculusDomainRelationalCalculusRelationalAlgebraProcedurallanguageSixbasicoperatorsselect:project:union:setdifference:–Cartesianproduct:xrename:Theoperatorstakeo

2、neortworelationsasinputsandproduceanewrelationasaresult.SelectOperation–ExampleRelationrA=B^D>5(r)SelectOperationNotation:p(r)piscalledtheselectionpredicateDefinedas:p(r)={t

3、trandp(t)}Wherepisaformulainpropositionalcalculusconsistingoftermsconnectedby:(and),(or),(not)

4、Eachtermisoneof:oporwhereopisoneof:=,,>,.<.Exampleofselection:dept_name=“Physics”(instructor)ProjectOperation–ExampleRelationr:A,C(r)ProjectOperationNotation:whereA1,A2areattributenamesandrisarelationname.Theresultisdefinedastherelationofk

5、columnsobtainedbyerasingthecolumnsthatarenotlistedDuplicaterowsremovedfromresult,sincerelationsaresetsExample:Toeliminatethedept_nameattributeofinstructorID,name,salary(instructor)UnionOperation–ExampleRelationsr,s:rs:UnionOperationNotation:rsDefinedas:rs={t

6、trorts}Forr

7、stobevalid.1.r,smusthavethesamearity(samenumberofattributes)2.Theattributedomainsmustbecompatible(example:2ndcolumnofrdealswiththesametypeofvaluesasdoesthe2ndcolumnofs)Example:tofindallcoursestaughtintheFall2009semester,orintheSpring2010semester,orinbothcourse_id(semester

8、=“Fall”Λyear=2009(section))course_id(semester=“Spring”Λyear=2010(section))SetdifferenceoftworelationsRelationsr,s:r–s:SetDifferenceOperationNotationr–sDefinedas:r–s={t

9、trandts}Setdifferencesmustbetakenbetweencompatiblerelations.randsmusthavethesamearityattributedomainsof

10、randsmustbecompatibleExample:tofindallcoursestaughtintheFall2009semester,butnotintheSpring2010semestercourse_id(semester=“Fall”Λyear=2009(section))−course_id(semester=“Spring”Λyear=2010(section))Cartesian-ProductOperation–ExampleRelationsr,s:rxs:Cartesian-ProductOperation

11、NotationrxsDefinedas:rxs={tq

12、trandqs}Assumethatattributesofr(R)and