标签：show 技术 dup color oss logs mysql sql ble
数据库查询中，涉及到auto_increment中的参数变量一共有两个
[root@localhost][(none)]> show variables like ‘auto_inc%‘; +--------------------------+-------+ | Variable_name | Value | +--------------------------+-------+ | auto_increment_increment | 1 | | auto_increment_offset | 1 | +--------------------------+-------+ 2 rows in set (0.00 sec)
auto_increment_increment：自增值
auto_increment_offset：漂移值,也就是步长
由于auto_increment_increment 属于全局可变的变量，故此可以通过修改自增值来达到测试目的
[root@localhost][(none)]> create table boss.autoinc1(col int not null auto_increment primary key); Query OK, 0 rows affected (1.03 sec) [root@localhost][(none)]> set @@auto_increment_increment=10; Query OK, 0 rows affected (0.00 sec) [root@localhost][(none)]> show variables like ‘auto_inc%‘; +--------------------------+-------+ | Variable_name | Value | +--------------------------+-------+ | auto_increment_increment | 10 | | auto_increment_offset | 1 | +--------------------------+-------+ 2 rows in set (0.00 sec)
从上面可以看到，自增从10开始，那么此时插入数据会是什么结果？
[root@localhost][(none)]> insert into boss.autoinc1 values(null),(null),(null),(null); Query OK, 4 rows affected (0.29 sec) Records: 4 Duplicates: 0 Warnings: 0 [root@localhost][(none)]> select col from boss.autoinc1; +-----+ | col | +-----+ | 1 | | 11 | | 21 | | 31 | +-----+ 4 rows in set (0.00 sec)
从结果集来看，auto_increment_increment的自增，为下一个跟上一个的间隔为10，也就是11->21->31->41以此类推
此时，我们设置offset这个的偏移值，那么数据则会
[root@localhost][(none)]> create table boss.autoinc2(col int not null auto_increment primary key); Query OK, 0 rows affected (1.31 sec) [root@localhost][(none)]> insert into boss.autoinc2 values(null),(null),(null),(null); Query OK, 4 rows affected (0.14 sec) Records: 4 Duplicates: 0 Warnings: 0 [root@localhost][(none)]> select col from boss.autoinc2; +-----+ | col | +-----+ | 5 | | 15 | | 25 | | 35 | +-----+ 4 rows in set (0.00 sec)
可以看到，第一个是从基数1偏移到5个值(1,2,3,4,5),然后自动增值，每次进10这么处理
本质的逻辑为 auto_increment_offset + N × auto_increment_increment N表示第几次，从0的技术开始计算
比如5+0*10,5+1*10，即
[root@localhost][mysql]> set @@auto_increment_offset=5; Query OK, 0 rows affected (0.00 sec) [root@localhost][mysql]> create table boss.autoinc6(col int not null auto_increment primary key); Query OK, 0 rows affected (0.36 sec) [root@localhost][mysql]> set @@auto_increment_increment=10; Query OK, 0 rows affected (0.00 sec) [root@localhost][mysql]> show variables like ‘auto_inc%‘; +--------------------------+-------+ | Variable_name | Value | +--------------------------+-------+ | auto_increment_increment | 10 | | auto_increment_offset | 5 | +--------------------------+-------+ 2 rows in set (0.00 sec) [root@localhost][mysql]> insert into boss.autoinc6 values(null),(null),(null),(null); Query OK, 4 rows affected (0.08 sec) Records: 4 Duplicates: 0 Warnings: 0 [root@localhost][mysql]> select col from boss.autoinc6; +-----+ | col | +-----+ | 5 | | 15 | | 25 | | 35 | +-----+ 4 rows in set (0.00 sec)
MySQL 中有关auto_increment及auto_increment_offset方面的介绍
标签：show 技术 dup color oss logs mysql sql ble
原文地址：http://www.cnblogs.com/olinux/p/6518766.html
