mysql> create table axa (i int);
Query OK,  0  rows affected ( 0 . 07  sec)

mysql> insert into axa values ( 920 );
Query OK,  1  row affected ( 0 . 02  sec)

mysql> alter table axa add z int primary key auto_increment;
Query OK,  1  row affected ( 0 . 01  sec)
Records:  1   Duplicates:  0   Warnings:  0 

mysql> select * from axa;
+ ------+---+
 
| i    | z |
+ ------+---+
 
|   920  |  1  |
+ ------+---+
 
 1  row in set ( 0 . 02  sec)