mysql> alter table tt change id id int default null;

Query OK, 1 row affected (0.23 sec)

Records: 1 Duplicates: 0 Warnings: 0

mysql> show create table tt;

+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------+

| Table | Create Table

|

+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------+

| tt | CREATE TABLE `tt` (

`id` int(11) NOT NULL DEFAULT "0",

`Field1` varchar(255) DEFAULT NULL,

PRIMARY KEY (`id`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8 |

+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------+

1 row in set (0.02 sec)

mysql> alter table tt modify id int not null auto_increment;

Query OK, 1 row affected (0.20 sec)

Records: 1 Duplicates: 0 Warnings: 0

mysql> select * from tt;

+----+--------+

| id | Field1 |

+----+--------+

| 1 | 華人 |

+----+--------+