What is the difference between UNION and UNION ALL?

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Accepted Answer

Both combine the result sets of two or more queries vertically (column count and types must match). UNION — combines and removes duplicates . Sorts internally to deduplicate, so it's slower. UNION ALL — combines and keeps duplicates . Faster because no deduplication is performed. Use UNION ALL by default; only use UNION when you specifically need distinct rows. UNION = removes duplicates (slower). UNION ALL = keeps all (faster). Always prefer UNION ALL when duplicates are acceptable (performance). Both require matching column count and types. ORDER BY applies to the final result.