设有关系R,S和T如下。关系T是由关系R和S经过______操作得到的。R S TABCABCABCa1b1c1a1b2c2a1b2c2a
设有关系R,S和T如下。关系T是由关系R和S经过______操作得到的。 R S T A B C A B C A B C a1 b1 c1 a1 b2 c2 a1 b2 c2 a1 b2 c2 a1 b3 c2 a2 b2 c1 a2 b2 c1 a2 b2 c1
A.R∪S
B.R-S
C.R×S
D.R∩S
设有关系R,S和T如下。关系T是由关系R和S经过______操作得到的。 R S T A B C A B C A B C a1 b1 c1 a1 b2 c2 a1 b2 c2 a1 b2 c2 a1 b3 c2 a2 b2 c1 a2 b2 c1 a2 b2 c1
A.R∪S
B.R-S
C.R×S
D.R∩S
A.P和SP
B.S和P
C.S和SP
D.S、P和SP
A.SELECT XH,GD FROM SC WHERE CH='3' ORDER BY GD DESC:
B.SELECT XH,GD FROM SC WHERE CH='3' ORDER BY GD ASC;
C.SELECT XH,GD FROM SC WHERE CH='3' GROUP BY GD DESC:
D.SELECT XH,GD FROM SC WHERE CH='3' GROUP BY GD ASC:
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