mysql> CREATE TABLE t1 (i INT);
mysql> explain select * from t1 where not exists ((select t11.i from t1 t11) union (select t12.i from t1 t12));
+----+--------------+------------+--------+---------------+------+---------+------+------+--------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+------------+--------+---------------+------+---------+------+------+--------------------------------+
| 1 | PRIMARY | t1 | system | NULL | NULL | NULL | NULL | 0 | const row not found |
| 2 | SUBQUERY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
| 3 | SUBQUERY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | no matching row in const table |
| 4 | UNION | t12 | system | NULL | NULL | NULL | NULL | 0 | const row not found |
| NULL | UNION RESULT | <union2,4> | ALL | NULL | NULL | NULL | NULL | NULL | |
+----+--------------+------------+--------+---------------+------+---------+------+------+--------------------------------+
|