MySQL auto_increment_increment，auto_increment_offset 用法

待我称王封你为后i 2022-08-13 00:37 193阅读 0赞

MySQL中对于表上ID自增列可以在创建表的时候来指定列上的auto\_increment属性；等同于SQL server中的identity属性；Oracle则是通过Sequence方式来实现。在MySQL中，系统变量auto\_increment\_increment，auto\_increment\_offset 影响自增列的值及其变化规则。本文主要描述这两个系统变量的相关用法。

1、auto\_increment\_increment与auto\_increment\_offset作用

auto_increment_increment控制列中的值的增量值，也就是步长。
    auto_increment_offset确定AUTO_INCREMENT列值的起点，也就是初始值。
    变量范围：可以在全局以及session级别设置这2个变量
    
    --当前系统环境
    root@localhost[(none)]> show variables like 'version';
    +---------------+------------+
    | Variable_name | Value      |
    +---------------+------------+
    | version       | 5.5.39-log |
    +---------------+------------+
    
    root@localhost[mysql]> create database tempdb;
    
    root@localhost[mysql]> use tempdb;
    
    --查看变量auto_increment_increment与auto_increment_offset
    root@localhost[tempdb]> show variables like '%auto_incre%';
    +--------------------------+-------+
    | Variable_name            | Value |
    +--------------------------+-------+
    | auto_increment_increment | 1     |
    | auto_increment_offset    | 1     |
    +--------------------------+-------+

2、演示auto\_increment\_increment与auto\_increment\_offset

--创建演示表，使用auto_increment子句
    root@localhost[tempdb]> create table t1(id int not null auto_increment primary key, col varchar(20));
    
    --插入记录
    root@localhost[tempdb]> insert into t1(col) values('robin'),('fred'),('jack'),('james');
    
    --下面可以看到id列起始值为1，增量为1
    root@localhost[tempdb]> select * from t1;
    +----+-------+
    | id | col   |
    +----+-------+
    |  1 | robin |
    |  2 | fred  |
    |  3 | jack  |
    |  4 | james |
    +----+-------+
    
    --设置步长为5
    root@localhost[tempdb]> set session auto_increment_increment=5;
    
    root@localhost[tempdb]> show variables like '%auto_incre%';
    +--------------------------+-------+
    | Variable_name            | Value |
    +--------------------------+-------+
    | auto_increment_increment | 5     |
    | auto_increment_offset    | 1     |
    +--------------------------+-------+
    
    --清空表t1
    root@localhost[tempdb]> truncate table t1;
    
    --再次插入记录
    root@localhost[tempdb]> insert into t1(col) values('robin'),('fred'),('jack'),('james');
    
    --如下查询可以看到步长以5位基数发生变化
    root@localhost[tempdb]> select * from t1;
    +----+-------+
    | id | col   |
    +----+-------+
    |  1 | robin |
    |  6 | fred  |
    | 11 | jack  |
    | 16 | james |
    +----+-------+
    
    --设置初始值为5
    root@localhost[tempdb]> set session auto_increment_offset=5;
    
    root@localhost[tempdb]> show variables like '%auto_incre%';
    +--------------------------+-------+
    | Variable_name            | Value |
    +--------------------------+-------+
    | auto_increment_increment | 5     |
    | auto_increment_offset    | 5     |
    +--------------------------+-------+
    
    root@localhost[tempdb]> truncate table t1;
    
    root@localhost[tempdb]> insert into t1(col) values('robin'),('fred'),('jack'),('james');
    
    --下面是新的结果
    root@localhost[tempdb]> select * from t1;
    +----+-------+
    | id | col   |
    +----+-------+
    |  5 | robin |
    | 10 | fred  |
    | 15 | jack  |
    | 20 | james |
    +----+-------+

3、auto\_increment\_increment与auto\_increment\_offset取值范围

--将变量auto_increment_increment设置为0
    root@localhost[tempdb]> set session auto_increment_increment=0;
    
    --实际值变成了1
    root@localhost[tempdb]> show variables like '%auto_increment%';
    +--------------------------+-------+
    | Variable_name            | Value |
    +--------------------------+-------+
    | auto_increment_increment | 1     |
    | auto_increment_offset    | 5     |
    +--------------------------+-------+
    
    --同样将auto_increment_offset设置为0
    root@localhost[tempdb]> set session auto_increment_offset=0;
    
    --实际值也变成了1
    root@localhost[tempdb]> show variables like '%auto_increment%';
    +--------------------------+-------+
    | Variable_name            | Value |
    +--------------------------+-------+
    | auto_increment_increment | 1     |
    | auto_increment_offset    | 1     |
    +--------------------------+-------+
    
    --下面尝试将2个变量设置为大于65535
    root@localhost[tempdb]> set session auto_increment_increment=65537;
    
    root@localhost[tempdb]> set session auto_increment_offset=65537;
    
    --其实际的值都变成了65535
    root@localhost[tempdb]> show variables like '%auto_increment%';
    +--------------------------+-------+
    | Variable_name            | Value |
    +--------------------------+-------+
    | auto_increment_increment | 65535 |
    | auto_increment_offset    | 65535 |
    +--------------------------+-------+
    
    --尝试为2个变量设置为负值
    root@localhost[tempdb]> set session auto_increment_offset=-2;
    
    root@localhost[tempdb]> set session auto_increment_increment=-5;
    
    --下面的查询可以看出全部恢复到缺省值1
    root@localhost[tempdb]> show variables like '%auto_increment%';
    +--------------------------+-------+
    | Variable_name            | Value |
    +--------------------------+-------+
    | auto_increment_increment | 1     |
    | auto_increment_offset    | 1     |
    +--------------------------+-------+
    
    由上可以看出2个变量只能设置为1至65535之间的整数值。
    所有非正整数全部会置为缺省值1，大于65535的值会被自动置为65535。

4、全局与session级别的设置

--查看全局范围这2个变量的值
    root@localhost[tempdb]> show global variables like '%auto_increment%';
    +--------------------------+-------+
    | Variable_name            | Value |
    +--------------------------+-------+
    | auto_increment_increment | 1     |
    | auto_increment_offset    | 1     |
    +--------------------------+-------+
    
    --下面分别设置session基本的值
    root@localhost[tempdb]> set session auto_increment_increment=5;
    
    root@localhost[tempdb]> set session auto_increment_offset=10;
    
    --查看session级别的值
    root@localhost[tempdb]> show session variables like '%auto_increment%';
    +--------------------------+-------+
    | Variable_name            | Value |
    +--------------------------+-------+
    | auto_increment_increment | 5     |
    | auto_increment_offset    | 10    |
    +--------------------------+-------+
    
    --查看全局级别的值
    root@localhost[tempdb]> show global variables like '%auto_increment%';
    +--------------------------+-------+
    | Variable_name            | Value |
    +--------------------------+-------+
    | auto_increment_increment | 1     |
    | auto_increment_offset    | 1     |
    +--------------------------+-------+
    
    --设置全局级别的值
    root@localhost[tempdb]> set global auto_increment_increment=2;
    
    root@localhost[tempdb]> set global auto_increment_offset=3;
    
    root@localhost[tempdb]> show global variables like '%auto_increment%';
    +--------------------------+-------+
    | Variable_name            | Value |
    +--------------------------+-------+
    | auto_increment_increment | 2     |
    | auto_increment_offset    | 3     |
    +--------------------------+-------+

5、已有auto\_increment列值任一变量变化的情形

root@localhost[tempdb]> truncate table t1;
    
    root@localhost[tempdb]> show variables like '%auto_increment%';
    +--------------------------+-------+
    | Variable_name            | Value |
    +--------------------------+-------+
    | auto_increment_increment | 1     |
    | auto_increment_offset    | 1     |
    +--------------------------+-------+
    
    root@localhost[tempdb]> insert into t1(col) values('robin'),('fred'),('jack');          
    
    root@localhost[tempdb]> select * from t1;
    +----+-------+
    | id | col   |
    +----+-------+
    |  1 | robin |
    |  2 | fred  |
    |  3 | jack  |
    +----+-------+
    
    root@localhost[tempdb]> set session auto_increment_increment=5;
    
    root@localhost[tempdb]> show variables like '%auto_increment%';
    +--------------------------+-------+
    | Variable_name            | Value |
    +--------------------------+-------+
    | auto_increment_increment | 5     |
    | auto_increment_offset    | 1     |
    +--------------------------+-------+
    
    --Author: Leshami
    --Blog  : http://blog.csdn.net/leshami
    
    root@localhost[tempdb]> insert into t1(col) values('david'),('tim'),('jerry');
    
    root@localhost[tempdb]> select * from t1;
    +----+-------+
    | id | col   |
    +----+-------+
    |  1 | robin |
    |  2 | fred  |
    |  3 | jack  |
    |  6 | david |
    | 11 | tim   |
    | 16 | jerry |
    +----+-------+
    
    New_value = auto_increment_offset+ N * auto_increment_increment
    New_value1 = 1 + 1 * 5 = 6
    New_value2 = 1 + 2 * 5 = 11
    
    --下面是修改auto_increment_offset后的结果
    root@localhost[tempdb]> set session auto_increment_offset=2;
    
    root@localhost[tempdb]> insert into t1(col) values('lewis'),('ian');
    
    root@localhost[tempdb]> select * from t1;
    +----+-------+
    | id | col   |
    +----+-------+
    |  1 | robin |
    |  2 | fred  |
    |  3 | jack  |
    |  6 | david |
    | 11 | tim   |
    | 16 | jerry |
    | 22 | lewis |
    | 27 | ian   |
    +----+-------+
    
    这个id为22，应该是这样推算来的：max(id)+(new_offset-old_offset)+increment
    也就是说变化auto_increment_offset后的第一个值为max(id)+(new_offset-old_offset)+increment之后再按步长递增。

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