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
