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Chapter2TheRelationalModelAdatabasemodel,ordatamodel,isasetofdefinitionsdescribinghowreal-worlddataisconceptuallyrepresentedascomputerizedinformation.Italsodescribesthetypesofoperationsavailabletoaccessandupdatethisinformation.AsexplainedinChapter1,thisbookconcentrateontherelationalmodel.Therelationa1modelistheclassicalonethathasbeeninusesincethelate1970s.Thereismuchtolearnabouthowvariousstructureswithinthetablearenamed(e.g.,theheadingofthetable),thestructuralrulestheyfollow(e.g.,tworowscannotbeidenticalinallcolumns),andanumberofotherconceptsusedindataaccessthatareramificationsoftheserules(e.g.,theprimarykeyforatableisasetofcolumnsthatidentifiesrowsuniquely).Thefirstfoursectionsofthischapterwillpresentsomestructuralconsiderationsoftherelationalmodel.StartingwithSection2.5,weintroducerelationalalgebra,acollectionoffundamentalrelationaloperationsthatcanbeusedtocreatenewtablesfromoldones.relationalrulesRe1ationalalgebraisanabstractlanguage,whichmeansthatitwillnotbepossibletoexecutequeriesformulatedinrelationalalgebraonanactualcomputer.Weintroducerelationalalgebrainthecurrentchapterinordertoexpress,inthesimplestform,thesetofoperationsthatanyrelationaldatabasequerylanguagemustperformtoanswerEng1ish-languagequeriesabouttheinformationinadatabase.Allofthesebasicconceptswillproveveryusefulinlaterchapters,whereweexplainthemeaningofvariousconstructsinSQL.HistoryofDatabaseSystems2.1TheCAPDatabaseAdatabaseisacollectionofcomputerizedrecordsmaintainedforacommonpurpose.Inthefollowingchapterswereferfrequentlytoaspecificexample,whichwecalltheCAPdatabase(forthetablenamesCUSTOMERS,AGENTS,andPRODUCTS).Thisdatabase,illustratedinFigure2.2,isusedbyawholesalebusinesstokeeptrackofitscustomers,theproductsitsellstothesecustomers,andtheagentswhoplaceordersforproductsonbehalfofthesecustomers.cidAGENTSPRODUCTSORDERSThetablesandcolumnsintheCAPdatabasearedescribedinFigure2.1.Thecustomerslistedarethemselvesretailbusinessesthatorderlargequantitiesofvariousproductsfromthewholesalecompanyforresal
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