What is the difference between union and union all in Oracle?

In Oracle, UNION and UNION ALL are used to combine the result sets of two or more SELECT statements. The difference between them is as follows:

1. UNION removes duplicate rows, while UNION ALL does not remove duplicate rows.
2. UNION ALL is more efficient than UNION because it does not require removing duplicate rows from the result set.
3. UNION requires sorting the result set to ensure there are no duplicate rows, while UNION ALL does not require sorting.
4. The number of rows in the result set returned by UNION ALL may be greater than the number of rows in the original result set because it includes duplicate rows.

Therefore, if you need to combine two query results without removing duplicate rows, you can use UNION ALL, which will be more efficient. If you need to remove duplicate rows, you can use UNION, but be aware that it may result in greater performance overhead.