●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 (19) 不能正确地查询出"零件号Pno等于'P3'的供应商名Sname",而 (20) 能正确查询的关系代数表达式。
WHERE EXISTS
(SELECT*
WHERE S.Sno=SPJ.SnO
WHERE 0<
(SEIECT COUNT(*)
WHERE S.Sno=SPJ.Sno
WHERE EXISTS
(SELECT*
WHERE S.Sno=SPJ.Sno
WHERES.Sno=SPJ.Sno