Databases · Junior · Question 31 of 75

What is the difference between `UNION` and `UNION ALL`?

1UNION joins tables side by side
2UNION ALL removes duplicates
✓Correct answer: UNION removes duplicate rows (extra work); UNION ALL keeps all rows

Explanation

Both stack the results of two queries with matching columns. Use UNION ALL when duplicates are impossible or wanted: it's faster.

Further reading

PostgreSQL: Combining queries (UNION, INTERSECT, EXCEPT) ↗

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