mysql> create table tbf (asset int, locale int);

mysql> describe tbf;
+--------+---------+------+-----+---------+-------+
| Field  | Type    | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+-------+
| asset  | int(11) | YES  |     | NULL    |       |
| locale | int(11) | YES  |     | NULL    |       |
+--------+---------+------+-----+---------+-------+
2 rows in set (0.30 sec)

mysql> insert into tbf values ( 1, null
Query OK, 1 row affected (0.25 sec)

mysql> insert into tbf values ( 1, 1 );
Query OK, 1 row affected (0.02 sec)

mysql> insert into tbf values ( 1, 2 );
Query OK, 1 row affected (0.04 sec)

mysql> insert into tbf values ( 2, 3 );
Query OK, 1 row affected (0.02 sec)

mysql> insert into tbf values ( 2, 4 );

mysql> select * from tbf;
+-------+--------+
| asset | locale |
+-------+--------+
|     1 |   NULL |
|     1 |      1 |
|     1 |      2 |
|     2 |      3 |
|     2 |      4 |
+-------+--------+
5 rows in set (0.04 sec)

mysql> select * from tbf t1 left join tbf t2 on ( t1.locale = t2.locale ) where t2.locale is null;
+-------+--------+-------+--------+
| asset | locale | asset | locale |
+-------+--------+-------+--------+
|     1 |   NULL |  NULL |   NULL |
+-------+--------+-------+--------+
1 row in set (0.24 sec)