MySQL 中有关auto_increment及auto_increment_offset方面的介绍

Posted 2020-09-03 熊出陌

数据库查询中，涉及到auto_increment中的参数变量一共有两个

[[email protected]][(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

[[email protected]][(none)]> create table boss.autoinc1(col int not null auto_increment primary key);
Query OK, 0 rows affected (1.03 sec)

[[email protected]][(none)]> set @@auto_increment_increment=10;
Query OK, 0 rows affected (0.00 sec)

[[email protected]][(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？

[[email protected]][(none)]> insert into boss.autoinc1 values(null),(null),(null),(null);
Query OK, 4 rows affected (0.29 sec)
Records: 4  Duplicates: 0  Warnings: 0

[[email protected]][(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

[[email protected]][(none)]> create table boss.autoinc2(col int not null auto_increment primary key);
Query OK, 0 rows affected (1.31 sec)

[[email protected]][(none)]> insert into boss.autoinc2 values(null),(null),(null),(null);
Query OK, 4 rows affected (0.14 sec)
Records: 4  Duplicates: 0  Warnings: 0

[[email protected]][(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

[[email protected]][mysql]> set @@auto_increment_offset=5;
Query OK, 0 rows affected (0.00 sec)

[[email protected]][mysql]> create table boss.autoinc6(col int not null auto_increment primary key);
Query OK, 0 rows affected (0.36 sec)

[[email protected]][mysql]> set @@auto_increment_increment=10;
Query OK, 0 rows affected (0.00 sec)

[[email protected]][mysql]> show variables like ‘auto_inc%‘;
+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| auto_increment_increment | 10    |
| auto_increment_offset    | 5     |
+--------------------------+-------+
2 rows in set (0.00 sec)

[[email protected]][mysql]> insert into boss.autoinc6 values(null),(null),(null),(null);
Query OK, 4 rows affected (0.08 sec)
Records: 4  Duplicates: 0  Warnings: 0

[[email protected]][mysql]> select col from boss.autoinc6;
+-----+
| col |
+-----+
|   5 |
|  15 |
|  25 |
|  35 |
+-----+
4 rows in set (0.00 sec)