MySQL 中有關auto_increment及auto_increment_offset方面的介紹

數據庫查詢中，涉及到auto_increment中的參數變量一共有兩個

[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,也就是步長

由於auto_increment_increment 屬於全局可變的變量，故此可以通過修改自增值來達到測試目的

[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)

從上面可以看到，自增從10開始，那么此時插入數據會是什么結果？

[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)

從結果集來看，auto_increment_increment的自增，為下一個跟上一個的間隔為10，也就是11->21->31->41以此類推

此時，我們設置offset這個的偏移值，那么數據則會

[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，第一個是從基數1偏移到5個值(1,2,3,4,5),然后自動增值，每次進10這么處理

本質的邏輯為  auto_increment_offset + N × auto_increment_increment  N表示第幾次，從0的技術開始計算

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)