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 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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