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。