Discussion
As per equivalence rules for query transformation, selection operation distributes over
Answer: D
Correct answer: All of the above.
Selection can be pushed through any of the three set operations, since deciding whether a tuple satisfies a condition does not depend on which relation it came from:
\[ \sigma_\theta(R \cup S) = \sigma_\theta(R) \cup \sigma_\theta(S) \]
and the same holds for intersection and difference. Doing the selection first is what makes such transformations worth applying.
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