假设供应商S和供应情况SPJ的关系模式分别为:S(Sno,Sname,Status,City)和SPJ(Sno,Pno,Jno,Qty)。SQ
假设供应商S和供应情况SPJ的关系模式分别为:S(Sno,Sname,Status,City)和SPJ(Sno,Pno,Jno,Qty)。SQL语句(22)不能正确地查询出“零件号Pno等于‘P3’的供应商名Sname”,而(23)能正确查询的关系代数表达式。
A.SELECT DISTINCT Sname FROMS WHERE EXISTS(SELECT* FROM SPJ WHERE S. Sno=SPJ. SnO AND SPJ.Pno='P3')
B.SELECT DISTmCT Sname FROMS WHERE 0<(SEIECT COUNT(*)FROM SPJ WHERES.Sno=SPJ.Sno AND SPJ.Pno='P3')
C.SELECT Sname FROMS WHERE EXISTS(SELECT* FROM SPJ WHERES.Sno=SPJ.Sno ANDSPJ.Pno='P3')
D.SELECT Sname FROMS,SPJ WHERES.Sno=SPJ.Sno ANDSPJ.Pno='P3')GROUP BYSname