问题详情

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(25)不能正确地查询出“零件号Pno等于"P3"的供应商名Sname"，而(26)能正确查询的关系代数表达式。
A．SELECT DISTINCT Sname FROM S WHERERE 0 ＜(SELECT COUNT(*)FROM SPJ WHERE S. Sno = SPJ. Sno AND SPJ. Pno = "P3")
B．SELECT DISTINCT Sname FROM S WHERE EXISTS(SELECT * FROM SPJ WHERE S. Sno=SPJ. Sno AND SPJ. Pno= "P3")
C．SELECT Sname FROM S WHERE EXISTS(SELECT * FROM SPJ WHERE S. Sno = SPJ. Sno AND SPJ. Pno = "P3")
D．SELECT Sname FROM S, SPJ WHERE S. Sno = SPJ. Sno AND SPJ. Pno = "P3" GROUP BY Sname请帮忙给出正确答案和分析，谢谢！