Difference between UNION and UNION ALL

Question

What is the difference between UNION and UNION ALL?

Answer 1

UNION removes NULL values when combining data, while UNION ALL leaves them.

Answer 2

When combining data, UNION ALL returns all rows, and UNION returns only unique rows.

Answer 3

UNION joins only those records that have matches, and UNION ALL joins all records without exception.

Answer 4

In UNION, the list of fields of the combined data must be identical, but in UNION ALL this is not necessary.