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 | 中国人
+----+--------+
