数据库原理与系统ch2

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1、Chapter2:RelationalModelChapter2:RelationalModelStructureofRelationalDatabasesFundamentalRelational-Algebra-OperationsAdditionalRelational-Algebra-OperationsExtendedRelational-Algebra-OperationsNullValuesModificationoftheDatabaseExampleofaRelationBasicStructureFormally,givensetsD1,D2,….Dnarel

2、ationrisasubsetofD1xD2x…xDnThus,arelationisasetofn-tuples(a1,a2,…,an)whereeachaiDiExample:Ifcustomer_name={Jones,Smith,Curry,Lindsay,…}/*Setofallcustomernames*/customer_street={Main,North,Park,…}/*setofallstreetnames*/customer_city={Harrison,Rye,Pittsfield,…}/*setofallcitynames*/Thenr={(Jone

3、s,Main,Harrison),(Smith,North,Rye),(Curry,North,Rye),(Lindsay,Park,Pittsfield)}isarelationovercustomer_namexcustomer_streetxcustomer_cityAttributeTypesEachattributeofarelationhasanameThesetofallowedvaluesforeachattributeiscalledthedomainoftheattributeAttributevaluesare(normally)requiredto

4、beatomic;thatis,indivisibleE.g.thevalueofanattributecanbeanaccountnumber,butcannotbeasetofaccountnumbersDomainissaidtobeatomicifallitsmembersareatomicThespecialvaluenullisamemberofeverydomainThenullvaluecausescomplicationsinthedefinitionofmanyoperationsWeshallignoretheeffectofnullvaluesinour

5、mainpresentationandconsidertheireffectlaterRelationSchemaA1,A2,…,AnareattributesR=(A1,A2,…,An)isarelationschemaExample:Customer_schema=(customer_name,customer_street,customer_city)r(R)denotesarelationrontherelationschemaRExample:customer(Customer_schema)RelationInstanceThecurrentvalues(relati

6、oninstance)ofarelationarespecifiedbyatableAnelementtofrisatuple,representedbyarowinatableJonesSmithCurryLindsaycustomer_nameMainNorthNorthParkcustomer_streetHarrisonRyeRyePittsfieldcustomer_citycustomerattributes(orcolumns)tuples(orrows)DATABASEProgrammingLanguagerelationschematypedefinition

7、relationvariablerelationinstancevalueofavariableRelationsareUnorderedOrderoftuplesisirrelevant(tuplesmaybestoredinanarbitraryorder)Example:accountrelationwithunorderedtuplesDatabaseAdatabaseconsistsofmultiplerelationsInformationaboutanenter