MySQL设置自动增长列

mysql> desc num;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| n     | int(11) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (2.59 sec)

mysql> alter table num change n n bigint not null auto_increment primary key;
Query OK, 3 rows affected (50.60 sec)
Records: 3  Duplicates: 0  Warnings: 0