[What is the role of auto_increment in MySQL?】

Directory

• • Source of problem
  • Explanation
  • Notes

Source of problem

Many times, the word [auto_increment] will appear in MySQL statements. Most of the time, tables are automatically generated. You will learn it when you first learn the MySQL database. Later, you may forget it gradually.What does effect mean?Here's a summary:

Explanation

auto_increment is used for primary key auto-increment, it starts to grow from 1, when you delete the first record and insert the second data, the primary key value is 2, not 1.

Example:

create table test

(

id int(10) not null auto_increment, – means auto-increment

name varchar(20) not null,

primary key(id)

)

auto_increment = 1; - Indicates the auto-increment starting size-- This way you can create a table test, the id is the auto-increment column

– execute the statement insert into test (name) values ​​('name');

– you can insert a row of data as: 1 'name'

Notes

When using AUTO_INCREMENT, the following points should be noted:

1, AUTO_INCREMENT is an attribute of the data column, only applicable to the integer type data column.

2, the data column for setting the AUTO_INCREMENT attribute should be a positive sequence, so the data column should be declared as UNSIGNED, so that the number of the sequence can be increased by onetimes.

3, the AUTO_INCREMENT data column must have a unique index to avoid repeated serial numbers (that is, the primary key or part of the primary key).
AUTO_INCREMENT data column must have NOT NULL attribute.

4, the maximum value of the serial number of the AUTO_INCREMENT data column is constrained by the data type of the column. For example, the maximum number of the TINYINT data column is 127, and if UNSIGNED is added, the maximum value is 255.Once the upper limit is reached, AUTO_INCREMENT will fail.

5. When a full table is deleted, MySQL AUTO_INCREMENT will start numbering from 1 again.

This is because when performing a full table operation, MySQL (the best combination with PHP) actually does this optimization operation: first delete all data and indexes in the data table,Then rebuild the data table.

If you want to delete all data rows and keep the sequence number information, you can use a delete command with where to suppress the optimization of MySQL (the best combination with PHP): delete from table_name where 1;

Use last_insert_id() to get the just incremented value.

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