浙江大学数据库系统概念PPT第二章,对应原版教材第五版.ppt

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1、RelationalModelStructureofRelationalDatabasesRelationalAlgebraFundamentalRelational-Algebra-OperationsAdditionalRelational-Algebra-OperationsExampleofaRelationRelationalDatabaseArelationaldatabaseconsistsofacollectionoftables.Eachtableisassignedauniquename.

2、Arowinatablerepresentsarelationshipamongasetofvalues.Atableisacollectionofsuchrelationship.Relationaldatamodelistheprimarydatamodelforcommercialdata-processingapplications.BecauseofitssimplicityEasyuseforprogrammerHardjobforDBMSsoftware.Databaseisacollectio

3、nofdataWhatisDataModel:Todescribe:Data,Datarelalationship,DataSemantices,DataConstraintsBasicStructureFormally,givensetsD1,D2,….DnarelationrisasubsetofD1xD2x…xDnThusarelationisasetofn-tuples(a1,a2,…,an)whereaiDiExample:ifcustomer-name={Jones,Smith,Curry,Li

4、ndsay}customer-street={Main,North,Park}customer-city={Harrison,Rye,Pittsfield}Thenr={(Jones,Main,Harrison),(Smith,North,Rye),(Curry,North,Rye),(Lindsay,Park,Pittsfield)}isarelationovercustomer-namexcustomer-streetxcustomer-cityAttributeTypesEachattribu

5、teofarelationhasanameThesetofallowedvaluesforeachattributeiscalledthedomainoftheattributeAttributevaluesare(normally)requiredtobeatomic,thatis,indivisibleE.g.thevalueofanattributecanbeanaccountnumber,butcannotbeasetofaccountnumbersDomainissaidtobeatomicifa

6、llitsmembersareatomicThespecialvaluenullisamemberofeverydomainThenullvaluecausescomplicationsinthedefinitionofmanyoperationsWeshallignoretheeffectofnullvaluesinourmainpresentationandconsidertheireffectlaterRelationSchemaA1,A2,…,AnareattributesR=(A1,A2,…,An)

7、isarelationschemaE.g.Customer-schema=(customer-name,customer-street,customer-city)r(R)isarelationontherelationschemaRE.g.customer(Customer-schema)RelationInstanceThecurrentvalues(relationinstance)ofarelationarespecifiedbyatableAnelementtofrisatuple,represe

8、ntedbyarowinatableJonesSmithCurryLindsaycustomer-nameMainNorthNorthParkcustomer-streetHarrisonRyeRyePittsfieldcustomer-citycustomerattributestuplesRelationsareUnorderedOrderoftuples(row)isirrelevant(ma