在数据表中会发现AUTO_INCREMENT的值不等于表中字段最大值+1，可能原因有以下几种：

如果步长不为1，则AUTO_INCREMENT=最大值+步长。关于步长不为1的参数说明，请参见14.1.62 表的自增AUTO_INCREMENT初值与步长。

mysql> show variables like 'auto_inc%'; 
+--------------------------+-------+ 
| Variable_name            | Value | 
+--------------------------+-------+ 
| auto_increment_increment | 2     | 
| auto_increment_offset    | 1     | 
+--------------------------+-------+ 
mysql> select * from auto_test1; 
+----+ 
| id | 
+----+ 
|  2 | 
|  4 | 
|  6 | 
|  8 | 
+----+ 
mysql> show create table auto_test1; 
+------------+-----------------------------------------+ 
| Table      | Create Table                            | 
+------------+-----------------------------------------+ 
| auto_test1 | CREATE TABLE `auto_test1` (   
`id` int NOT NULL AUTO_INCREMENT,   
PRIMARY KEY (`id`) 
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8 | 
+------------+-----------------------------------------+

直接修改表的AUTO_INCREMENT，会导致AUTO_INCREMENT变化。

mysql> select * from animals; 
+----+-----------+ 
| id | name      | 
+----+-----------+ 
|  1 | fish       | 
|  2 | cat       | 
|  3 | penguin   | 
+----+-----------+ 
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=4 DEFAULT CHARSET=utf8           | 
+---------+-----------------------------------------------------+ 
mysql> alter table animals AUTO_INCREMENT=100; 
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=100 DEFAULT CHARSET=utf8         | 
+---------+-----------------------------------------------------+

未提交的事务或回滚的事务，会导致AUTO_INCREMENT增长，但回滚后不会下降。

mysql> show create table auto_test1; 
+------------+----------------------------------------+ 
| Table      | Create Table                           | 
+------------+----------------------------------------+ 
| auto_test1 | CREATE TABLE `auto_test1` (   
`id` int NOT NULL AUTO_INCREMENT,   
PRIMARY KEY (`id`) 
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 | 
+------------+----------------------------------------+ 
1 row in set (0.00 sec)  
mysql> select * from auto_test1; 
+----+ 
| id | 
+----+ 
|  1 | 
|  2 | 
|  3 | 
+----+ 
mysql> begin; 
Query OK, 0 rows affected (0.02 sec)  
mysql> insert into auto_test1 values (0),(0),(0); 
Query OK, 3 rows affected (0.00 sec) 
Records: 3  Duplicates: 0  Warnings: 0  
mysql> select * from auto_test1; 
+----+ 
| id | 
+----+ 
|  1 | 
|  2 | 
|  3 | 
|  4 | 
|  5 | 
|  6 | 
+----+ 
6 rows in set (0.00 sec)  
mysql> show create table auto_test1; 
+------------+----------------------------------------+ 
| Table      | Create Table                           | 
+------------+----------------------------------------+ 
| auto_test1 | CREATE TABLE `auto_test1` (   
`id` int NOT NULL AUTO_INCREMENT,   
PRIMARY KEY (`id`) 
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8 | 
+------------+----------------------------------------+ 
1 row in set (0.00 sec)  
mysql> rollback; 
Query OK, 0 rows affected (0.05 sec)  
mysql> select * from auto_test1; 
+----+ 
| id | 
+----+ 
|  1 | 
|  2 | 
|  3 | 
+----+ 
3 rows in set (0.00 sec)  
mysql> show create table auto_test1; 
+------------+----------------------------------------+ 
| Table      | Create Table                           | 
+------------+----------------------------------------+ 
| auto_test1 | CREATE TABLE `auto_test1` (   
`id` int NOT NULL AUTO_INCREMENT,   
PRIMARY KEY (`id`) 
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8 | 
+------------+----------------------------------------+

数据插入后，AUTO_INCREMENT变化，然后删除对应的数据行，AUTO_INCREMENT不会下降。

mysql> show create table auto_test1; 
+------------+----------------------------------------+ 
| Table      | Create Table                           | 
+------------+----------------------------------------+ 
| auto_test1 | CREATE TABLE `auto_test1` (   
`id` int NOT NULL AUTO_INCREMENT,   
PRIMARY KEY (`id`) 
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 | 
+------------+----------------------------------------+ 
1 row in set (0.00 sec)  
mysql> select * from auto_test1; 
+----+ 
| id | 
+----+ 
|  1 | 
|  2 | 
|  3 | 
+----+ 
mysql> insert into auto_test1 values (0),(0),(0); 
Query OK, 3 rows affected (0.00 sec) 
Records: 3  Duplicates: 0  Warnings: 0  
mysql> select * from auto_test1; 
+----+ 
| id | 
+----+ 
|  1 | 
|  2 | 
|  3 | 
|  4 | 
|  5 | 
|  6 | 
+----+ 
6 rows in set (0.00 sec)  
mysql> show create table auto_test1; 
+------------+----------------------------------------+ 
| Table      | Create Table                           | 
+------------+----------------------------------------+ 
| auto_test1 | CREATE TABLE `auto_test1` (   
`id` int NOT NULL AUTO_INCREMENT,   
PRIMARY KEY (`id`) 
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8 | 
+------------+----------------------------------------+ 
1 row in set (0.00 sec)  
mysql> delete from auto_test1 where id>3;  
mysql> select * from auto_test1; 
+----+ 
| id | 
+----+ 
|  1 | 
|  2 | 
|  3 | 
+----+ 
3 rows in set (0.00 sec)  mysql> show create table auto_test1; 
+------------+----------------------------------------+ 
| Table      | Create Table                           | 
+------------+----------------------------------------+ 
| auto_test1 | CREATE TABLE `auto_test1` (   
`id` int NOT NULL AUTO_INCREMENT,   
PRIMARY KEY (`id`) 
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8 | 
+------------+----------------------------------------+

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