How to use the auto-increment function in MySQL

MySQL is one of the most popular open source databases currently. Its powerful functions and ease of use also make it popular. Among them, auto-increment is one of the commonly used functions in MySQL. This article will introduce how MySQL uses the auto-increment function.

1. What is auto-increment

Auto-increment is an automatic growth function in the MySQL database. When there is an auto-increment field in the table, each time a new piece of data is inserted, the auto-increment field will automatically increment by 1. Auto-increment can solve some problems very well, such as automatic ID generation, automatic sequence generation, etc.

2. How to set auto-increment fields

To set auto-increment, you need to add the attribute AUTO_INCREMENT to the relevant fields when creating the table. For example:

CREATE TABLE user (
id int(11) NOT NULL AUTO_INCREMENT,
name varchar(30) NOT NULL,
PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

In the above statement, we set the id as an auto-increment field, Its type is int and length is 11. The AUTO_INCREMENT option tells MySQL to automatically assign a new value to the id field every time a piece of data is inserted.

To add an auto-increment field to an already created table, you need to use the ALTER TABLE statement. For example:

ALTER TABLE user ADD COLUMN age INT(11) NOT NULL AUTO_INCREMENT AFTER name;

In this In the example, we added an auto-increment field named age. Note that when adding an auto-increment field, make sure the field is NOT NULL.

3. How to insert an auto-increment field

When inserting an auto-increment field, we can look like this:

INSERT INTO user ( name) VALUES ('Zhang San');

Querying this table can yield:

SELECT * FROM user;

id	name
1	张三

You can see that even if we do not assign a value to id in the statement, the id field is automatically incremented. This is because there is AUTO_INCREMENT in the field properties. This attribute tells MySQL that when data is inserted into the table, MySQL will automatically assign a unique value to the id field so that it will automatically increment.

If we insert another piece of data, as follows:

INSERT INTO user (name) VALUES ('李思');

Querying this table can yield:

SELECT * FROM user;

id	name
1	张三
2	李四

You can see that the id is automatically incremented again to 2.

4. Limitations of auto-increment fields

While using auto-increment fields, we also need to understand some limitations of auto-increment:

1. Auto-increment Fields can only be of integer type, not decimal, text, etc.
2. The auto-increment field value is unique in the table, that is, there can only be one auto-increment field in a table, and it can only be PRIMARY KEY or UNIQUE KEY.
3. Once the auto-increment field is used, it cannot be redesigned, otherwise you will encounter problems such as data loss.

5. Summary

Auto-increment is a commonly used function in MySQL table design, and it can also solve some problems well in actual development. This article describes how MySQL generates auto-increment fields through the Auto_Increment attribute, and how to use auto-increment fields to increment sequence values ​​when inserting data. At the same time, we also need to pay attention to the limitations of auto-increment fields and some precautions.

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