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What is the difference between UNION and UNION ALL? Which one is faster and why?

SQLmedium0.5 min readPremium
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487
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1,863
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Aarete
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Expert AnswerPremium
103 wordsInterview-ready
UNION combines results of two queries and removes duplicates (like DISTINCT). UNION ALL keeps all rows, including duplicates. UNION ALL is faster: no deduplication sort/hash. Use UNION when duplicates must be removed; UNION ALL when you know sources are disjoint or duplicates are acceptable. Example: SELECT a FROM t1 UNION ALL SELECT a FROM t2. Best practice: Prefer UNION ALL unless dedup is required; it scales better and is clearer about intent....
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