How to add auto-increment attributes to fields in mysql

In mysql, you can add auto-increment attributes to the field by adding the "AUTO_INCREMENT" attribute to the field. The syntax is "alter table table name add column field name data type AUTO_INCREMENT;".

The operating environment of this tutorial: windows7 system, mysql8 version, Dell G3 computer.

Achieve field auto-growth by adding the AUTO_INCREMENT attribute to the field. The syntax format is as follows:

字段名 数据类型 AUTO_INCREMENT

By default, the initial value of AUTO_INCREMENT is 1. Every time a new record is added, the field value is automatically increased by 1.

• Only one field in a table can use the AUTO_INCREMENT constraint, and the field must have a unique index to avoid repeated serial numbers (that is, the primary key or part of the primary key).

• The fields of the AUTO_INCREMENT constraint must have the NOT NULL attribute.

• AUTO_INCREMENT constrained fields can only be of integer type (TINYINT, SMALLINT, INT, BIGINT, etc.).

• AUTO_INCREMENT The maximum value of a constrained field is constrained by the data type of the field. If the upper limit is reached, AUTO_INCREMENT will become invalid.

Auto-increment fields are generally used in primary keys.

When the primary key is defined as self-increasing, the value of the primary key no longer requires the user to enter data, but is automatically assigned by the database system according to the definition. Each time a record is added, the primary key will automatically increase with the same step size.

Example:

Existing mysql data table, I hope to add an auto-increment field and set the initial value of the new data.

• It’s actually not complicated, just a reminder.

Test table

CREATE TABLE `t_abc` (
  `name` varchar(20) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Test data:

INSERT INTO `t_abc` (`name`)
VALUES
	('mike'),
	('tom'),
	('jack');

Add auto-increment fields and set the starting value of new data

/*增加一个自增主键字段，分两步操作*/

/*首先增加自增字段*/
alter table t_abc add column id int auto_increment primary key;
/*执行完上面这一条，字段增加，但值为空*/

/*执行这一条，它会自动为已存在的数据的自增字段赋初值，从1开始，同时将后续新增的数据从100开始*/
alter table t_abc auto_increment=100;

Modify existing ones Data initial value

/*如果希望所有的数据都从10001 开始，我们可以这么做*/

alter table t_abc add column id int auto_increment primary key;

/*这里没有指定任何数值，执行完后只为自增字段赋从1开始的初始值，其实隐含的设置当前表自增字段从1开始*/
alter table t_abc auto_increment;

/*将所有数据增加10000*/
update t_abc set id=id+10000;

/* 前面的100 我们是任意指定的，现在我们应该指定数据库中的maxId+1作为下一个数据的起始值*/
set @maxId=1;
select max(id) into @maxId from t_abc; /*表中有3条数据，那么maxId 现在是10003*/
select @maxId+1 from dual; /* 10004 */

alter table t_abc auto_increment=10004; /*这里不能直接引用变量，因此手动挪移过来*/

Verify it

insert into t_abc(name) values('Marry');
select * from t_abc order by id desc;

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