--给表增加自增字段的时候报错

mysql> desc t7;
+-------+-------------+------+-----+---------+-------+
| Field | Type| Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| name | varchar(20) | YES | | NULL||
| id| int(4) | YES | | NULL||
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> alter table t7 add id2 int(4) auto_increment;
ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key

报错原因：自增字段没有被定义为一个key。

解决办法：可以将新增字段定义为unique key或primary key。

mysql> alter table t7 add id2 int(4) auto_increment primary key;
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> alter table t7 drop column id2;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> alter table t7 add id2 int(4) auto_increment unique key;
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0