单选

●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。
(19) 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 WHERE S.Sno=SPJ.Sno
AND SPJ.Pno=′P3′)
C．SELECT Sname
FROMS WHERE EXISTS (SELECT*
FROM SPJ WHERE S.Sno=SPJ.Sno
AND SPJ.Pno=′P3′)
D．SELECT Sname
FROMS，SPJ WHERES.Sno=SPJ.Sno
AND SPJ.Pno=′P3′)
GROUP BYSname (20) A．Πsname(S)Dσ 0 Pno=′P3′(SPJ)
B．Πsname(SD∞σ Pno=′P3′(SPJ))
C．Πsname(S)-Πsname(σ Pno≠′P3′(Sσσ(SPJ)
D．Πsname(S)-Πsname((S∞σ Pno≠′P3′(SPJ)

试题出自试卷《2011年网络管理员考试考前密卷（二）-上午试题》

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