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Explain Union vs Union All in SQL.

SQLmedium0.4 min readPremium
Frequency
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Asked at 1 company
Category
487
questions in SQL
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130E|271M|86H
in this category
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1,863
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Gartner
Key Concepts Tested
joinsql
Expert AnswerPremium
79 wordsInterview-ready
**Architectural Logic**: UNION deduplicates; UNION ALL does not—semantic and performance differ. **UNION**: Combines; removes duplicates (implicit DISTINCT); slower (sort). **UNION ALL**: Combines; keeps all rows; faster. **Use UNION**: When dedup required. **Use UNION ALL**: When impossible or acceptable. **Example**: Disjoint date ranges = UNION ALL correct and faster. **Scalability**: UNION ALL avoids shuffle for dedup; UNION is O(n log n). **Cost**: UNION adds compute; know source data....
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