beginnerSet Operators — UNION, INTERSECT, EXCEPT/MINUS

What's the difference between UNION and UNION ALL, and which should you default to?

UNION combines both result sets and removes duplicate rows, requiring a sort or hash pass over the entire combined output. UNION ALL combines them with no deduplication at all, which is strictly cheaper. Default to UNION ALL unless you specifically know duplicates can occur and need to be removed.

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