第四讲 第2章 关系数据库(关系代数2)ppt课件.ppt

《第四讲 第2章 关系数据库(关系代数2)ppt课件.ppt》由会员上传分享，免费在线阅读，更多相关内容在教育资源-天天文库。

1、数据库技术及应用TechnologiesandApplicationsofDatabases华中科技大学水电与数字化工程学院第二章关系数据库RelationalDatabasesLecture4关系代数RelationalAlgebraLearningObjectivesEvolutionandimportanceofrelationalmodelTermsinrelationalmodelliketuple,domain,cardinality,anddegreeofarelationOperationsinrelatio

2、nalalgebraRelationalAlgebraProcedurallanguageSixbasicoperatorsselect:project:union:setdifference:–Cartesianproduct:xrename:Theoperatorstakeoneortworelationsasinputsandproduceanewrelationasaresult.SelectOperation–ExampleRelationrA=B^D>5(r)SelectOperationNotatio

3、n:p(r)piscalledtheselectionpredicateDefinedas:p(r)={t

4、trandp(t)}Wherepisaformulainpropositionalcalculusconsistingoftermsconnectedby:(and),(or),(not)Eachtermisoneof:oporwhereopisoneof:=,,>,.<.Exampleofselection:dept_name=“P

5、hysics”(instructor)ProjectOperation–ExampleRelationr:A,C(r)ProjectOperationNotation:whereA1,A2areattributenamesandrisarelationname.TheresultisdefinedastherelationofkcolumnsobtainedbyerasingthecolumnsthatarenotlistedDuplicaterowsremovedfromresult,sincerelationsares

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

7、trorts}Forrstobevalid.1.r,smusthavethesamearity(samenumberofattributes)2.Theattributedomainsmustbe

8、compatible(example:2ndcolumnofrdealswiththesametypeofvaluesasdoesthe2ndcolumnofs)Example:tofindallcoursestaughtintheFall2009semester,orintheSpring2010semester,orinbothcourse_id(semester=“Fall”Λyear=2009(section))course_id(semester=“Spring”Λyear=2010(section)

9、)SetdifferenceoftworelationsRelationsr,s:r–s:SetDifferenceOperationNotationr–sDefinedas:r–s={t

10、trandts}Setdifferencesmustbetakenbetweencompatiblerelations.randsmusthavethesamearityattributedomainsofrandsmustbecompatibleExample:tofindallcoursestaughtintheFall2009s

11、emester,butnotintheSpring2010semestercourse_id(semester=“Fall”Λyear=2009(section))−course_id(semester=“Spring”Λyear=2010(section))Cartesian-