一道SQL考试题.多重嵌套类型,非高手勿进.
设供应商零件的关系模式为SP(Sno,Pno,Qty),其中Sno表示供应商号,Pno表示零件号,Qty表示零件数量。查询至少包含了供应商“168”所供应的全部零件的供应商号的SQL语句如下:
SELECT Sno
FROM SP SPX
WHERE __ (19) __
(SELECT *
FROM SP SPY
WHERE __ (20) __ AND NOT EXISTS
(SELECT *
FROM SP SPZ
WHERE __ (21) __));
(19) A.EXISTS B. NOT EXISTS C. IN D. NOT IN
(20) A. SPY.Sno=’168’ B. SPY Sno<>’168’
C.SPY.Sno=SPX.Sno D.SPY.Sno<>SPX.Sno
(21) A.SPZ.Sno=SPY.Sno AND SPZ.Pno=SPY.Pno
B. SPZ.Sno=SPX.Sno AND SPZ.Pno=SPX.Pno
C. SPZ.Sno=SPX.Sno AND SPZ.Pno=SPY.Pno
D. SPY.Sno=’168’ AND SPZ.Pno=SPY.Pno
答案为BAC,请问21问中SPX,SPY,SPZ代表的表的内容是什么?是怎样计算出来的?有请各位高手帮帮忙