UNION vs UNION ALL in MySQL: Differences and Examples

UNION and UNION ALL both stack result sets vertically, but they treat duplicates differently — and that difference has a real performance cost that analysts should understand.

Key Points

• UNION removes duplicate rows; UNION ALL keeps everything
• UNION ALL is faster — no deduplication sort is needed
• Both require the same number of columns with compatible types
• Column names come from the first SELECT
• ORDER BY applies to the combined result, at the end
• Use UNION ALL by default; switch to UNION only when duplicates must go