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Explain SQL set operations (UNION/INTERSECT/EXCEPT) and NULL handling pitfalls.

Set operations and NULL semantics trip up even senior engineers. The signal is UNION vs UNION ALL and its dedup cost, plus three-valued logic where NULL equals nothing, not even itself.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.

Set operations and NULL semantics trip up even senior engineers. The signal is UNION vs UNION ALL and its dedup cost, plus three-valued logic where NULL equals nothing, not even itself.

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