练习 9 已知关系R和S,求RUS,R∩S,RS,R×S R B C D E g d d An Introduction to Database SystemAn Introduction to Database System 练习 R S A B C D E F a b c d g a d a f d a f c b d 已知关系R和S,求R∪S，R∩S，R-S，R×S