📊 Day 6 – Data Analyst Most Asked Interview Question ❓

UNION vs UNION ALL (SQL)

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UNION

• Combines result sets
• Removes duplicate rows
• Slightly slower due to deduplication
• Columns count & data types must match

UNION ALL

• Combines result sets
• Keeps duplicates
• Faster than UNION
• Columns count & data types must match

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Rule:

👉 Duplicates should be removed → UNION
👉 Performance matters & duplicates allowed → UNION ALL ✅

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