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 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> exit Bye [2@a data]$ mysql -uroot -pwsdad Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 5 Server 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 its affiliates. Other names may be trademarks of their respective owners. Type ‘help;‘ or ‘\h‘ for help. Type ‘\c‘ to clear the current input statement. mysql> use wison Database changed mysql> delete from test_innodb where id=4; Query OK, 1 row affected (0.07 sec) mysql> exit Bye [2@a data]$ sudo service mysql restart Shutting down MySQL... SUCCESS! Starting MySQL.. SUCCESS! [2@a data]$ mysql -uroot -pwison Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 1 Server 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 its affiliates. Other names may be trademarks of their respective owners. Type ‘help;‘ or ‘\h‘ for help. Type ‘\c‘ to clear the current input statement. mysql> use wison Database changed mysql> 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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