Повторяю еще раз: приведи пример!
У меня для любой таблицы все работает замечательно.
mysql> create table u1(
-> id int);
Query OK, 0 rows affected (0.16 sec)
mysql> create table u2(
-> id int);
Query OK, 0 rows affected (0.01 sec)
mysql> insert into u1 values(1),(2),(3);
Query OK, 3 rows affected (0.04 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> insert into u2 values(4),(5),(6);
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> (select * from u1);
+------+
| id |
+------+
| 1 |
| 2 |
| 3 |
+------+
3 rows in set (0.00 sec)
mysql> (select * from u1) union (select * from u2);
+------+
| id |
+------+
| 1 |
| 2 |
| 3 |
| 4 |
| 5 |
| 6 |
+------+
6 rows in set (0.15 sec)
