What is the difference between UNION and UNION ALL?
What is the difference between UNION and UNION ALL?
UNION
removes duplicate records (where all columns in the results are the same), UNION ALL
does not.
There is a performance hit when using UNION
instead of UNION ALL
, since the database server must do additional work to remove the duplicate rows, but usually you do not want the duplicates (especially when developing reports).
UNION Example:
SELECT 'foo' AS bar UNION SELECT 'foo' AS bar
Result:
+-----+
| bar |
+-----+
| foo |
+-----+
1 row in set (0.00 sec)
UNION ALL example:
SELECT 'foo' AS bar UNION ALL SELECT 'foo' AS bar
Result:
+-----+
| bar |
+-----+
| foo |
| foo |
+-----+
2 rows in set (0.00 sec)
SELECT 'foo' AS bar UNION SELECT 'foo' AS bar; SELECT 'foo' AS bar UNION ALL SELECT 'foo' AS bar;
查询结果如下