1. Meaning
Identity column,
also known as self-increasing column.
Meaning: There is no need to manually insert values, and the system provides default sequence values.
Two, usage
一、创建表时设置标识列
CREATE TABLE tab_identity(
id INT PRIMARY KEY,
NAME VARCHAR(20)
);
INSERT INTO tab_identity
VALUES(1,'john');
CREATE TABLE tab_identity2(
id INT PRIMARY KEY AUTO_INCREMENT,
NAME VARCHAR(20)
);
INSERT INTO tab_identity2
VALUES(NULL,'john');
这样也可以
VALUES('john');
SELECT * FROM tab_identity2;
SHOW VARIABLES LIKE '%auto_increment%';
SET auto_increment_increment = 3;
更改步长
更改起始值：
INSERT INTO tab_identity2
VALUES(10,'john');
其他的用NULL
则从10开始。
1.标识列一定要和主键或唯一等key搭配。
2.一个表中只能有一个标识列
3.标识列的类型只能是数值型
4.标识列可以通过SET auto_increment_increment = 3;
更改步长，
也可以更改起始值：
INSERT INTO tab_identity2
VALUES(10,'john');
修改表设置标识列
ALTER TABLE xxx MODIFY COLUMN id INT PRIMARY KEY AUTO_INCREMENT;
修改表时删除标识列
ALTER TABLE xxx MODIFY COLUMN id INT;Figure 1 Self-growth effect
Figure 2 The value of self-increment, the first is the step size, and the second is the starting value, which can be changed. See above
