mysql> desc one
    -> ;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   | PRI | NULL    |       |
| empno | int(11) | NO   | UNI | NULL    |       |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

alter table one drop index unique_key_name;