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For $R \cup S$ to be a valid relational-algebra expression, $R$ and $S$ must
Ahave the same number of tuples
Bshare at least one common primary key value
Chave identical column shape (same attributes)
Dbe stored on the same physical disk
Answer & Solution
Correct answer: C. have identical column shape (same attributes)
Pooling two boxes only works if both boxes hold the same kind of items. Set operations on relations (union, intersection, difference) all need both inputs to have identical column shape: same attribute names and types, in the same order. Tuple counts and storage location do not matter.
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