mysql> (select 'a') union (select 'a');
+---+
| a |
+---+
| a |
+---+
1 row in set (0.00 sec)
mysql> (select 'a') union all (select 'a');
+---+
| a |
+---+
| a |
| a |
+---+
2 rows in set (0.00 sec)
mysql> (select 'a') union distinct (select 'a');
+---+
| a |
+---+
| a |
+---+
1 rows in set (0.00 sec)
+---+
| a |
+---+
| a |
+---+
1 row in set (0.00 sec)
mysql> (select 'a') union all (select 'a');
+---+
| a |
+---+
| a |
| a |
+---+
2 rows in set (0.00 sec)
mysql> (select 'a') union distinct (select 'a');
+---+
| a |
+---+
| a |
+---+
1 rows in set (0.00 sec)
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