# 修改表的自增AUTO_INCREMENT值
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
  ```