我有下面的表模式，它将user_customers映射到一个活跃的MySQL数据库的权限:

mysql> describe user_customer_permission;
+------------------+---------+------+-----+---------+----------------+
| Field            | Type    | Null | Key | Default | Extra          |
+------------------+---------+------+-----+---------+----------------+
| id               | int(11) | NO   | PRI | NULL    | auto_increment |
| user_customer_id | int(11) | NO   | PRI | NULL    |                |
| permission_id    | int(11) | NO   | PRI | NULL    |                |
+------------------+---------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

我想删除user_customer_id和permission_id的主键，并保留id的主键。

执行命令时:

alter table user_customer_permission drop primary key;

我得到以下错误:

ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key

如何删除列的主键?