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

SQLmedium0.5 min readPremium

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...

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Frequency
Low
Asked at 1 company
Category
487
questions in SQL
Difficulty Split
130E|271M|86H
in this category
Total Bank
1,863
across 7 categories
Asked at these companies
Aarete
Key Concepts Tested
join

Why This Question Matters

This medium-level SQL question appears frequently in data engineering interviews at companies like Aarete. While less common, it tests deeper understanding that distinguishes strong candidates. Mastering the underlying concepts (join) will help you answer variations of this question confidently.

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Expert Answer
103 words

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. Why it matters: Design choices compound at scale—wrong approach can cause 100× overhead. Scalability trade-offs: Profile before optimizing; validate on sample then full. Cost implications: Suboptimal choices multiply at billion-row scale.

The complete answer continues with detailed implementation patterns, architectural trade-offs, and production-grade considerations covering performance optimization and real-world examples.

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