Verify that selections commute—that is, for any relation r and any pair of selection conditions cond 1 and cond 2 , ? cond 1 (? cond 2 (r)) = ? cond 2 (? cond 1 (r)).
What will be an ideal response?
?cond1(?cond2(r)) = ?cond1 AND cond2(r))
?cond2(?cond1(r)) = ?cond1 AND cond2(r))
Thus, ?cond1(?cond2(r)) = ?cond2(?cond1(r)).
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