mysql> create table zc_test2(id int);Query OK, 0 rows affected (1.37 sec)mysql> insert into zc_test2 values (10),(9),(8),(7),(6);Query OK, 5 rows affected (0.04 sec)Records: 5  Duplicates: 0  Warnings: 0mysql> select * from zc_test2;+------+| id   |+------+|   10 ||    9 ||    8 ||    7 ||    6 |+------+5 rows in set (0.00 sec)mysql> ALTER TABLE zc_test2 add id2 int;Query OK, 0 rows affected (2.84 sec)Records: 0  Duplicates: 0  Warnings: 0mysql> select * from zc_test2;+------+------+| id   | id2  |+------+------+|   10 | NULL ||    9 | NULL ||    8 | NULL ||    7 | NULL ||    6 | NULL |+------+------+5 rows in set (0.00 sec)mysql> alter table zc_test2 change id2 id2 int NOT NULL AUTO_INCREMENT;ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a keymysql> alter table zc_test2 change id2 id2 int NOT NULL AUTO_INCREMENT primary key;Query OK, 5 rows affected (1.34 sec)Records: 5  Duplicates: 0  Warnings: 0mysql> select * from zc_test2;+------+-----+| id   | id2 |+------+-----+|   10 |   1 ||    9 |   2 ||    8 |   3 ||    7 |   4 ||    6 |   5 |+------+-----+5 rows in set (0.00 sec)