What is the difference between UNION and UNION ALL?

Main difference between UNION and UNION ALL is that UNION removes duplicate records, but UNION ALL does not remove duplicate records.

E.g. Consider two tables A and B
A B
10 15
20 20

UNION of A and B = 10, 20, 15
UNION ALL of A and B = 10, 12, 15, 20

Performance of UNION ALL is considered better than UNION, since UNION ALL does not require additional work of removing duplicates.

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