UNION vs UNION ALL

Question

UNION vs UNION ALL

Accepted Answer

UNION performs a deduplication step after stacking the two result sets, similar to running DISTINCT on the combined output. UNION ALL skips that step and returns every row from both queries, even if some rows are identical. UNION ALL is almost always faster because deduplication requires sorting or hashing the combined result - prefer it when you know duplicates cannot exist or when you need to count them.