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1、WorkingwithDatabasesThischapterdescribeshowtouseSQLstatementsinembeddedapplicationstocontroldatabases.Therearethreedatabasestatementsthatsetupandopendatabasesforaccess:SETDATABASEdeclaresadatabasehandle,associatesthehandlewithanactualdatabasefile,andoptionallyassignsoperation
2、alparametersforthedatabase.SETNAMESoptionallyspecifiesthecharactersetaclientapplicationusesforCHAR,VARCHAR,andtextBlobdata.Theserverusesthisinformationtotransliteratefromadatabase’sdefaultcharactersettotheclient’scharactersetonSELECToperations,andtotransliteratefromaclientapp
3、lication’scharactersettothedatabasecharactersetonINSERTandUPDATEoperations.gCONNECTopensadatabase,allocatessystemresourcesforit,andoptionallyassignsoperationalparametersforthedatabase.Alldatabasesmustbeclosedbeforeaprogramends.AdatabasecanbeclosedbyusingDISCONNECT,orbyappendi
4、ngtheRELEASEoptiontothefinalCOMMITorROLLBACKinaprogram.DeclaringadatabaseBeforeadatabasecanbeopenedandusedinaprogram,itmustfirstbedeclaredwithSETDATABASEto:CHAPTER3WORKINGWITHDATABASES.Establishadatabasehandle.Associatethedatabasehandlewithadatabasefilestoredonalocalorremoten
5、ode.Adatabasehandleisaunique,abbreviatedaliasforanactualdatabasename.DatabasehandlesareusedinsubsequentCONNECT,COMMITRELEASE,andROLLBACKRELEASEstatementstospecifywhichdatabasestheyshouldaffect.ExceptindynamicSQL(DSQL)applications,databasehandlescanalsobeusedinsidetransactionb
6、lockstoqualify,ordifferentiate,tablenameswhentwoormoreopendatabasescontainidenticallynamedtables.Eachdatabasehandlemustbeuniqueamongallvariablesusedinaprogram.Databasehandlescannotduplicatehost-languagereservedwords,andcannotbeInterBasereservedwords.Thefollowingstatementillus
7、tratesasimpledatabasedeclaration:EXECSQLSETDATABASEDB1=’employee.gdb’;Thisdatabasedeclarationidentifiesthedatabasefile,employee.gdb,asadatabasetheprogramuses,andassignsthedatabaseahandle,oralias,DB1.Ifaprogramrunsinadirectorydifferentfromthedirectorythatcontainsthedatabasefil
8、e,thenthefilenamespecificationinSETDATABASEmustincludeafullpathname,
