Explain SQL set operations (UNION/INTERSECT/EXCEPT) and NULL handling pitfalls.
Set operations and NULL semantics catch out even senior engineers. The signal is UNION vs UNION ALL and its dedup cost, plus three-valued logic where NULL matches nothing, not even itself.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
Set operations and NULL semantics catch out even senior engineers. The signal is UNION vs UNION ALL and its dedup cost, plus three-valued logic where NULL matches nothing, not even itself.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
