●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 (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。

(25) 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

(26) A．∏Sname(S) σPno=′P3′(SPJ)

B．∏Sname(S σPno=′P3′(SPJ))

C．∏Sname(S)-∏Sname(S σPno=′P3′(SPJ))

D．∏Sname(S)-∏Sname(σPno=′P3′(S (SPJ)))