What is the difference between UNION and UNION ALL?

Point	UNION	UNION ALL
Duplicates	Removed	Kept
Speed	Slower (has to find duplicates)	Faster

SELECT city FROM students
UNION
SELECT city FROM students;       -- 4 rows

SELECT city FROM students
UNION ALL
SELECT city FROM students;       -- 12 rows

Both queries must have the same number of columns. See MySQL UNION.

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