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1、WorkingwithDatabasesThischapterdescribeshowtouseSQLstatementsinembeddedapplicationstocontroldatabases.Therearethreedatabasestatementsthatsetupandopendatabasesforaccess:SETDATABASEdeclaresadatabasehandle,associatesthehandlewithanactualdatabasefile,andopt
2、ionallyassignsoperationalparametersforthedatabase.SETNAMESoptionallyspecifiesthecharactersetaclientapplicationusesforCHAR,VARCHAR,andtextBlobdata.Theserverusesthisinformationtotransliteratefromadatabase’sdefaultcharactersettotheclient’scharactersetonSEL
3、ECToperations,andtotransliteratefromaclientapplication’scharactersettothedatabasecharactersetonINSERTandUPDATEoperations.gCONNECTopensadatabase,allocatessystemresourcesforit,andoptionallyassignsoperationalparametersforthedatabase.Alldatabasesmustbeclose
4、dbeforeaprogramends.AdatabasecanbeclosedbyusingDISCONNECT,orbyappendingtheRELEASEoptiontothefinalCOMMITorROLLBACKinaprogram.DeclaringadatabaseBeforeadatabasecanbeopenedandusedinaprogram,itmustfirstbedeclaredwithSETDATABASEto:CHAPTER3WORKINGWITHDATABASES
5、.Establishadatabasehandle.Associatethedatabasehandlewithadatabasefilestoredonalocalorremotenode.Adatabasehandleisaunique,abbreviatedaliasforanactualdatabasename.DatabasehandlesareusedinsubsequentCONNECT,COMMITRELEASE,andROLLBACKRELEASEstatementstospecif
6、ywhichdatabasestheyshouldaffect.ExceptindynamicSQL(DSQL)applications,databasehandlescanalsobeusedinsidetransactionblockstoqualify,ordifferentiate,tablenameswhentwoormoreopendatabasescontainidenticallynamedtables.Eachdatabasehandlemustbeuniqueamongallvar
7、iablesusedinaprogram.Databasehandlescannotduplicatehost-languagereservedwords,andcannotbeInterBasereservedwords.Thefollowingstatementillustratesasimpledatabasedeclaration:EXECSQLSETDATABASEDB1=’employee.gdb’;Thisdatabasedeclarationidentifiesthedatabasef
8、ile,employee.gdb,asadatabasetheprogramuses,andassignsthedatabaseahandle,oralias,DB1.Ifaprogramrunsinadirectorydifferentfromthedirectorythatcontainsthedatabasefile,thenthefilenamespecificationinSETDATABASEmustincludeafullpathname,
