# 创建关于表t，其中a字段为主键自增
mysql> create table t(a bigint primary key auto_increment, b tinyint);
Query OK, 0 rows affected (0.03 sec)
# 插入一些数据
mysql> insert into t select null, 10;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> insert into t select null, 20;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> insert into t select null, 30;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> insert into t select null, 40;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
# 查看表记录
mysql> select * from t;
+---+------+
| a | b |
+---+------+
| 1 | 10 |
| 2 | 20 |
| 3 | 30 |
| 4 | 40 |
+---+------+
4 rows in set (0.00 sec)
# 删除最后一条数据
mysql> delete from t where a=4;
Query OK, 1 row affected (0.02 sec)
# 查看表创建语句，发现AUTO_INCREMENT=5
mysql> show create table t\G
*************************** 1. row ***************************
Table: t
Create Table: CREATE TABLE `t` (
`a` bigint(20) NOT NULL AUTO_INCREMENT,
`b` tinyint(4) DEFAULT NULL,
PRIMARY KEY (`a`)
) ENGINE=InnoDB
AUTO_INCREMENT=5
DEFAULT CHARSET=latin1
1 row in set (0.00 sec)
# 进行主键回溯模拟
# 重启数据库
[root@mysql ~]# service mysqld restart
Shutting down MySQL.. SUCCESS!
Starting MySQL. SUCCESS!
# 重新查看表创建语句，发现AUTO_INCREMENT=4
mysql> show create table t\G
*************************** 1. row ***************************
Table: t
Create Table: CREATE TABLE `t` (
`a` bigint(20) NOT NULL AUTO_INCREMENT,
`b` tinyint(4) DEFAULT NULL,
PRIMARY KEY (`a`)
) ENGINE=InnoDB
AUTO_INCREMENT=4
DEFAULT CHARSET=latin1
1 row in set (0.00 sec)
# 继续插入语句
mysql> insert into t select null, 50;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
# 查看表的数据，发现上述自增ID=4又重新出现
mysql> select * from t;
+---+------+
| a | b |
+---+------+
| 1 | 10 |
| 2 | 20 |
| 3 | 30 |
|
4
| 50 |
+---+------+
4 rows in set (0.00 sec)
这是因为在MySQL5.7中的表的AUTO_INCREMENT是基于内存，不会持久化在磁盘中，每次启动数据库时，会对每张表进行max(auto_increment) + 1重新作为该表下一次的主键ID的自增值。在MySQL8.0中就不会出现该问题，因为数据会在磁盘中持久化。
来自 “ ITPUB博客 ” ，链接：https://blog.itpub.net/29812844/viewspace-2658344/ ，如需转载，请注明出处，否则将追究法律责任。
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