--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
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