修改方法如下：

当AUTO_INCREMENT大于表中数据的最大值时，可以在取值范围内任意修改为更大的值。

mysql> show create table animals; 
+---------+------------------------------------------------------------------+ 
| Table   | Create Table                                                     | 
+---------+------------------------------------------------------------------+ 
| animals | CREATE TABLE `animals` (   
`id` mediumint NOT NULL AUTO_INCREMENT,   `name` char(30) NOT NULL,   
PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=101 DEFAULT CHARSET=utf8   | 
+---------+------------------------------------------------------------------+ 
1 row in set (0.00 sec)  
mysql> select * from animals; 
+-----+-----------+ 
| id  | name      | 
+-----+-----------+ 
| -50 | -middle   | 
|   1 | fish       | 
|   2 | cat       | 
|  50 | middle    | 
| 100 | rabbit    | 
+-----+-----------+ 
11 rows in set (0.00 sec)  
mysql> alter table animals AUTO_INCREMENT=200; 
Query OK, 0 rows affected (0.22 sec) 
Records: 0  Duplicates: 0  Warnings: 0  
mysql> show create table animals; 
+---------+-------------------------------------------------------+ 
| Table   | Create Table                                          | 
+---------+-------------------------------------------------------+ 
| animals | CREATE TABLE `animals` (   
`id` mediumint NOT NULL AUTO_INCREMENT,   `name` char(30) NOT NULL,   
PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=200 DEFAULT CHARSET=utf8 | 
+---------+-------------------------------------------------------+

当AUTO_INCREMENT大于表中数据的最大值时，如果修改后的指定值仍大于数据的最大值，则修改为指定值成功。否则，默认会修改为数据最大值+1。

mysql> select * from animals; 
+-----+-----------+ 
| id  | name      | 
+-----+-----------+ 
| -50 | -middle   | 
|   1 | fish       | 
|   2 | cat       | 
|  50 | middle    | 
| 100 | rabbit    | 
+-----+-----------+ 
mysql> show create table animals; 
+---------+-----------------------------------------------------+ 
| Table   | Create Table                                        | 
+---------+-----------------------------------------------------+ 
| animals | CREATE TABLE `animals` (   
`id` mediumint NOT NULL AUTO_INCREMENT,   `name` char(30) NOT NULL,   
PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=200 DEFAULT CHARSET=utf8 | 
+---------+-----------------------------------------------------+ 
mysql> alter table animals AUTO_INCREMENT=150; 
Query OK, 0 rows affected (0.05 sec) 
Records: 0  Duplicates: 0  Warnings: 0 
mysql> show create table animals; 
+---------+-----------------------------------------------------+ 
| Table   | Create Table                                         | 
+---------+-----------------------------------------------------+ 
| animals | CREATE TABLE `animals` (   
`id` mediumint NOT NULL AUTO_INCREMENT,   `name` char(30) NOT NULL,   
PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=150 DEFAULT CHARSET=utf8 | 
+---------+-----------------------------------------------------+ 
mysql> alter table animals AUTO_INCREMENT=50; 
Query OK, 0 rows affected (0.04 sec) 
Records: 0  Duplicates: 0  Warnings: 0 
mysql> show create table animals; 
+---------+-----------------------------------------------------+ 
| Table   | Create Table                                        | 
+---------+-----------------------------------------------------+ 
| animals | CREATE TABLE `animals` (   
`id` mediumint NOT NULL AUTO_INCREMENT,   `name` char(30) NOT NULL,   
PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=101 DEFAULT CHARSET=utf8 | 
+---------+-----------------------------------------------------+ 
mysql> delete from animals where id=100; 
Query OK, 1 row affected (0.00 sec)  
mysql> select * from animals; 
+-----+-----------+ 
| id  | name      | 
+-----+-----------+ 
| -50 | -middle   | 
|   1 | fish       | 
|   2 | cat       | 
|  50 | middle    | 
+-----+-----------+ 
10 rows in set (0.00 sec)  
mysql> alter table animals AUTO_INCREMENT=50; 
Query OK, 0 rows affected (0.04 sec) 
Records: 0  Duplicates: 0  Warnings: 0  
mysql> show create table animals; 
+---------+-----------------------------------------------------+ 
| Table   | Create Table                                        | 
+---------+-----------------------------------------------------+ 
| animals | CREATE TABLE `animals` (   
`id` mediumint NOT NULL AUTO_INCREMENT,   `name` char(30) NOT NULL,   
PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=51 DEFAULT CHARSET=utf8 | 
+---------+-----------------------------------------------------+ 
1 row in set (0.00 sec)

AUTO_INCREMENT无法修改为负数。

mysql> alter table animals AUTO_INCREMENT=-1; 
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '-1' at line 1

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