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1、WorkingwithDatabasesThischapterdescribeshowtouseSQLstatementsinembeddedapplicationstocontroldatabases.Therearethreedatabasestatementsthatsetupandopendatabasesforaccess:SETDATABASEdeclaresadatabasehandle,associatesthehandlewithanactualdatabasefile,andoptionallyassignsoperationalparametersforthedat
2、abase.SETNAMESoptionallyspecifiesthecharactersetaclientapplicationusesforCHAR,VARCHAR,andtextBlobdata.Theserverusesthisinformationtotransliteratefromadatabase’sdefaultcharactersettotheclient’scharactersetonSELECToperations,andtotransliteratefromaclientapplication’scharactersettothedatabasecharact
3、ersetonINSERTandUPDATEoperations.gCONNECTopensadatabase,allocatessystemresourcesforit,andoptionallyassignsoperationalparametersforthedatabase.Alldatabasesmustbeclosedbeforeaprogramends.AdatabasecanbeclosedbyusingDISCONNECT,orbyappendingtheRELEASEoptiontothefinalCOMMITorROLLBACKinaprogram.Declarin
4、gadatabaseBeforeadatabasecanbeopenedandusedinaprogram,itmustfirstbedeclaredwithSETDATABASEto:CHAPTER3WORKINGWITHDATABASES.Establishadatabasehandle.Associatethedatabasehandlewithadatabasefilestoredonalocalorremotenode.Adatabasehandleisaunique,abbreviatedaliasforanactualdatabasename.Databasehandles
5、areusedinsubsequentCONNECT,COMMITRELEASE,andROLLBACKRELEASEstatementstospecifywhichdatabasestheyshouldaffect.ExceptindynamicSQL(DSQL)applications,databasehandlescanalsobeusedinsidetransactionblockstoqualify,ordifferentiate,tablenameswhentwoormoreopendatabasescontainidenticallynamedtables.Eachdata
6、basehandlemustbeuniqueamongallvariablesusedinaprogram.Databasehandlescannotduplicatehost-languagereservedwords,andcannotbeInterBasereservedwords.Thefollowingstatementillustratesasimpledatabasedeclaration:EXECSQLSETDATABASEDB1=’employee.gdb’;Thisdatabasedeclarationidentifiesthedatabasefile,employe
7、e.gdb,asadatabasetheprogramuses,andassignsthedatabaseahandle,oralias,DB1.Ifaprogramrunsinadirectorydifferentfromthedirectorythatcontainsthedatabasefile,thenthefilenamespecificationinSETDATABASEmustincludeafullpathname,
