mysql> USE pets;
Database changed

mysql> ALTER TABLE dog ADD pet_id int(11);
Query OK, 1 row affected (0.35 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> ALTER TABLE dog MODIFY column pet_id int(11)
    -> auto_increment primary key;
Query OK, 1 row affected (0.03 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> DESCRIBE dog;
+--------+-------------+------+-----+---------+----------------+
| Field  | Type        | Null | Key | Default | Extra          |
+--------+-------------+------+-----+---------+----------------+
| name   | varchar(20) | YES  |     | NULL    |                | 
| owner  | varchar(20) | YES  |     | NULL    |                | 
| breed  | varchar(20) | YES  |     | NULL    |                | 
| sex    | char(1)     | YES  |     | NULL    |                | 
| birth  | date        | YES  |     | NULL    |                | 
| death  | date        | YES  |     | NULL    |                | 
| pet_id | int(11)     | NO   | PRI | NULL    | auto_increment | 
+--------+-------------+------+-----+---------+----------------+
7 rows in set (0.02 sec)

mysql> SELECT * FROM dog;
+------+-----------+-------+------+------------+------------+--------+
| name | owner     | breed | sex  | birth      | death      | pet_id |
+------+-----------+-------+------+------------+------------+--------+
| Fido | Mr. Jones | Mutt  | M    | 2004-11-12 | 2006-04-02 |      1 | 
| Lassie | Tommy Reting | Collie  | F    | 2006-01-10 | NULL |      2 | 
+------+-----------+-------+------+------------+------------+--------+
2 row in set (0.00 sec)
