表的自动新增AUTOINCREMENT超过数据中该字段的最大值

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：

1.步长不为1，则AUTO_INCREMENT=最大值+步长。

mysql> show variables like 'auto_inc%';
+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| auto_increment_increment | 10    |
| auto_increment_offset    | 5     |
+--------------------------+-------+
2 rows in set (0.00 sec)
mysql> select * from tianyiy;
+-----+
| uid |
+-----+
|   5 |
|  15 |
|  25 |
+-----+
3 rows in set (0.00 sec)
mysql> show create table tianyiy;
+---------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                                          |
+---------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tianyiy | CREATE TABLE `tianyiy` (
  `uid` int(11) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`uid`)
) ENGINE=InnoDB AUTO_INCREMENT=35 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+---------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

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

mysql> select * from tianyiyun;
+-----+------+
| uid | user |
+-----+------+
| -30 | k    |
|   1 | a    |
|   2 | b    |
|   3 | c    |
|   4 | d    |
|   5 | e    |
|   6 | f    |
|   7 | g    |
|   8 | h    |
|  30 | j    |
|  99 | i    |
+-----+------+
11 rows in set (0.00 sec)
mysql> show create table tianyiyun;
+-----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table     | Create Table                                                                                                                                                                                                                  |
+-----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tianyiyun | CREATE TABLE `tianyiyun` (
  `uid` mediumint(9) NOT NULL AUTO_INCREMENT,
  `user` char(10) COLLATE utf8mb4_bin NOT NULL,
  PRIMARY KEY (`uid`)
) ENGINE=InnoDB AUTO_INCREMENT=100 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+-----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> alter table tianyiyun AUTO_INCREMENT=99;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> show create table tianyiyun;
+-----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table     | Create Table                                                                                                                                                                                                                  |
+-----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tianyiyun | CREATE TABLE `tianyiyun` (
  `uid` mediumint(9) NOT NULL AUTO_INCREMENT,
  `user` char(10) COLLATE utf8mb4_bin NOT NULL,
  PRIMARY KEY (`uid`)
) ENGINE=InnoDB AUTO_INCREMENT=100 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+-----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

3.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 tiayiyun_test;
+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table         | Create Table                                                                                                                                                               |
+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tiayiyun_test | CREATE TABLE `tiayiyun_test` (
  `uid` int(11) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`uid`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> select * from tiayiyun_test;
+-----+
| uid |
+-----+
|   1 |
|   2 |
|   3 |
|   4 |
|   5 |
|   6 |
+-----+
6 rows in set (0.00 sec)
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> insert into tiayiyun_test values (0),(0),(0);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0
mysql> select * from tiayiyun_test;
+-----+
| uid |
+-----+
|   1 |
|   2 |
|   3 |
|   4 |
|   5 |
|   6 |
|  15 |
|  25 |
|  35 |
+-----+
9 rows in set (0.00 sec)
mysql> show create table tiayiyun_test;
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table         | Create Table                                                                                                                                                                |
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tiayiyun_test | CREATE TABLE `tiayiyun_test` (
  `uid` int(11) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`uid`)
) ENGINE=InnoDB AUTO_INCREMENT=45 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> rollback;
Query OK, 0 rows affected (0.01 sec)
mysql> select * from tiayiyun_test;
+-----+
| uid |
+-----+
|   1 |
|   2 |
|   3 |
|   4 |
|   5 |
|   6 |
+-----+
6 rows in set (0.00 sec)
mysql> show create table tiayiyun_test;
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table         | Create Table                                                                                                                                                                |
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tiayiyun_test | CREATE TABLE `tiayiyun_test` (
  `uid` int(11) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`uid`)
) ENGINE=InnoDB AUTO_INCREMENT=45 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

4.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 tiayiyun_test;
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table         | Create Table                                                                                                                                                                |
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tiayiyun_test | CREATE TABLE `tiayiyun_test` (
  `uid` int(11) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`uid`)
) ENGINE=InnoDB AUTO_INCREMENT=45 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> select * from tiayiyun_test;
+-----+
| uid |
+-----+
|   1 |
|   2 |
|   3 |
|   4 |
|   5 |
|   6 |
+-----+
6 rows in set (0.00 sec)
mysql> insert into tiayiyun_test values (0),(0),(0);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0
mysql> select * from tiayiyun_test;
+-----+
| uid |
+-----+
|   1 |
|   2 |
|   3 |
|   4 |
|   5 |
|   6 |
|  45 |
|  55 |
|  65 |
+-----+
9 rows in set (0.00 sec)
mysql> show create table tiayiyun_test;
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table         | Create Table                                                                                                                                                                |
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tiayiyun_test | CREATE TABLE `tiayiyun_test` (
  `uid` int(11) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`uid`)
) ENGINE=InnoDB AUTO_INCREMENT=75 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> delete from tiayiyun_test where uid>3;
Query OK, 6 rows affected (0.00 sec)
mysql> select * from tiayiyun_test;
+-----+
| uid |
+-----+
|   1 |
|   2 |
|   3 |
+-----+
3 rows in set (0.00 sec)
mysql> show create table tiayiyun_test;
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table         | Create Table                                                                                                                                                                |
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tiayiyun_test | CREATE TABLE `tiayiyun_test` (
  `uid` int(11) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`uid`)
) ENGINE=InnoDB AUTO_INCREMENT=75 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)