Mysql中Identity 详细介绍

假如表中包含一列为auto_increment，

如果是Myisam类型的引擎，那么在删除了最新一笔数据，无论是否重启Mysql，下一次插入之后仍然会使用上次删除的最大ID+1.

mysql> create table test_myisam (id int not null auto_increment primary key, name char(5)) engine=myisam;Query OK, 0 rows affected (0.04 sec)  mysql> insert into test_myisam (name) select ‘a‘;Query OK, 1 row affected (0.00 sec)Records: 1 Duplicates: 0 Warnings: 0  mysql> insert into test_myisam (name) select ‘b‘;Query OK, 1 row affected (0.00 sec)Records: 1 Duplicates: 0 Warnings: 0  mysql> insert into test_myisam (name) select ‘c‘;Query OK, 1 row affected (0.00 sec)Records: 1 Duplicates: 0 Warnings: 0  mysql> insert into test_myisam (name) select name from test_myisam;Query OK, 3 rows affected (0.00 sec)Records: 3 Duplicates: 0 Warnings: 0  mysql> select * from test_myisam;+----+------+| id | name |+----+------+| 1 | a  || 2 | b  || 3 | c  || 4 | a  || 5 | b  || 6 | c  |+----+------+6 rows in set (0.00 sec)  mysql> delete from test_myisam where id=6;Query OK, 1 row affected (0.00 sec)

mysql> insert into test_myisam(name) select ‘d‘;Query OK, 1 row affected (0.00 sec)Records: 1 Duplicates: 0 Warnings: 0  mysql> select * from test_myisam;+----+------+| id | name |+----+------+| 1 | a  || 2 | b  || 3 | c  || 4 | a  || 5 | b  || 7 | d  |+----+------+6 rows in set (0.00 sec)

下面是对Innodb表的测试。

mysql> create table test_innodb(id int not null auto_increment primary key, name char(5)) engine=innodb;Query OK, 0 rows affected (0.26 sec)  mysql> insert into test_innodb (name)select ‘a‘;Query OK, 1 row affected (0.06 sec)Records: 1 Duplicates: 0 Warnings: 0  mysql> insert into test_innodb (name)select ‘b‘;Query OK, 1<b>本文来源gao@dai!ma.com搞$代^码!网7</b> row affected (0.06 sec)Records: 1 Duplicates: 0 Warnings: 0  mysql> insert into test_innodb (name)select ‘c‘;Query OK, 1 row affected (0.07 sec)Records: 1 Duplicates: 0 Warnings: 0  mysql> select * from test_innodb;+----+------+| id | name |+----+------+| 1 | a  || 2 | b  || 3 | c  |+----+------+3 rows in set (0.00 sec)  mysql> delete from test_innodb where id=3;Query OK, 1 row affected (0.05 sec)  mysql> insert into test_innodb (name)select ‘d‘;Query OK, 1 row affected (0.20 sec)Records: 1 Duplicates: 0 Warnings: 0  mysql> select * from test_innodb;+----+------+| id | name |+----+------+| 1 | a  || 2 | b  || 4 | d  |+----+------+3 rows in set (0.00 sec)  mysql> exitBye[2@a data]$ mysql -uroot -pwsdadWelcome to the MySQL monitor. Commands end with ; or \g.Your MySQL connection id is 5Server version: 5.5.37-log Source distribution  Copyright (c) 2000, 2014, Oracle and/or its affiliates. All rights reserved.  Oracle is a registered trademark of Oracle Corporation and/or itsaffiliates. Other names may be trademarks of their respectiveowners.  Type ‘help;‘ or ‘\h‘ for help. Type ‘\c‘ to clear the current input statement.  mysql> use wisonDatabase changedmysql> delete from test_innodb where id=4;Query OK, 1 row affected (0.07 sec)  mysql> exitBye[2@a data]$ sudo service mysql restartShutting down MySQL... SUCCESS!Starting MySQL.. SUCCESS![2@a data]$ mysql -uroot -pwisonWelcome to the MySQL monitor. Commands end with ; or \g.Your MySQL connection id is 1Server version: 5.5.37-log Source distribution  Copyright (c) 2000, 2014, Oracle and/or its affiliates. All rights reserved.  Oracle is a registered trademark of Oracle Corporation and/or itsaffiliates. Other names may be trademarks of their respectiveowners.  Type ‘help;‘ or ‘\h‘ for help. Type ‘\c‘ to clear the current input statement.  mysql> use wisonDatabase changedmysql> insert into test_innodb (name) select ‘z‘;Query OK, 1 row affected (0.07 sec)Records: 1 Duplicates: 0 Warnings: 0  mysql> select * from test_innodb;+----+------+| id | name |+----+------+| 1 | a  || 2 | b  || 3 | z  |+----+------+3 rows in set (0.00 sec)

可以看到在mysql数据库没有重启时，innodb的表新插入数据会是之前被删除的数据再加1.

但是当Mysql服务被重启后，再向InnodB的自增表表里插入数据，那么会使用当前Innodb表里的最大的自增列再加1.

原因：

Myisam类型存储引擎的表将最大的ID值是记录到数据文件中，不管是否重启最大的ID值都不会丢失。但是InnoDB表的最大的ID值是存在内存中的，若不重启Mysql服务，新加入数据会使用内存中最大的数据+1.但是重启之后，会使用当前表中最大的值再+1

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