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1、空值查询~量词谓词（Nullquery~quantifierpredicate）*identifyingpurchaseordersnotyetdeterminedbysuppliersSelect*fromsupplierisnull(supplyfirm)=.T.Select*fromsupplierwhereisnull(supplyfirm)=.T.USEf:fourthchaptercommonlyusedtablesupplier.DbfEXCLUSIVECloseallUSEf:fourthchaptercommonlyusedtables
2、upplier.DbfEXCLUSIVEBROWSELASTSelect*fromorderformwhereisnull(supplyfirm)=.T.Select*fromorderformwheresupplyfirmisnull*inVF,.NULL.isusedtorepresentnullvalues;InSQL,nullvaluesarerepresentedbyisnull*notethatwhenyoudenynullvaluesinsideSQL,youuseisnotnullinsteadofisn'tnull.Select*fromord
3、ersheet,supplyfirmisn'tnullSelect*fromordersheet,supplyfirmisnotnullSelect*fromorderformwheresupplyfirmisnotnullSelect*fromorderformwheresupplyfirmisn'tnullSelectwarehousenumber,area,salary,fromwarehouse,staffwhere,employeenumber=E3"Selectwarehouse.Warehousenumber,area,salary,fromwar
4、ehouse,staffwhere,employeenumber=E3"Selectwarehouse.Warehousenumber,area,salary,fromwarehouse,staffwhere,employeenumber=E3,and;Warehouse.Warehousenumber=worker.WarehouseNo.SelectA.warehousenumber,area,salary,fromwarehouse,a,staffbwhere,employeenumber=E3,and;A.warehousenumber=b.wareho
5、usenumberSelectA.warehousenumber,area,salary,fromwarehouse,asa,staffas,bwhere,employeenumber=E3,and;A.warehousenumber=b.warehousenumber*settingaliasescanuse"as"orspaces*function:simplifyoperation,;Formultipletables,connectionqueriesSelectgy.employeename,asmanager,leader,jl.employeena
6、me,asemployee,fromemployee,asGy,employeeasjl;Wheregy.employeenumber=jl.ManagerSelectgy.employeename,asmanager,leadership,asrelationship,jl.employeename,asemployee,fromemployee,asGy,employeeasjl;Wheregy.employeenumber=jl.Manager*employeeasGy,employeeasJL:atablewith2aliasesistoturnatab
7、leintotwotablesforuseinselfconnectionThe"leading"stringthatappearsafter*select,whichisinsertedintothequeryresultandisreferredtoas"leadership""Selectjl.employeename,"lead",gy.employeename,fromemployee,asGy,employeeasjl;Wherejl.manager,=gy.employeenumber*betweenthetwotables,whopointsto
8、whoisdetermi
