Talk about auto in MySQL in detail_ What is the function of increment

Catalog

Source of problem

explain

matters needing attention

summary

Source of problem

A lot of times ,MySQL In the statement 【auto_increment】 This word , Most of the time , Tables are automatically generated , Just beginning to learn MySQL Database will learn , later , Gradually you may forget , What exactly does this sentence mean ？ below , Let's summarize ：

explain

auto_increment It is used for automatic growth of primary keys , from 1 Start to grow , When you delete the first record , When you insert the second data , The primary key value is 2, No 1.

for example ：

create table test(id int(10) not null auto_increment, – Indicates auto increment name varchar(20) not null,primary key(id))

auto_increment = 1; – Indicates the starting size of self increment -- So you can create a table test,id For the column

– Execute statement insert into test (name) values (‘ name ’);

– You can insert a row of data as : 1 ‘ name ’

matters needing attention

In the use of AUTO_INCREMENT when , We should pay attention to the following points ：

1、AUTO_INCREMENT Is an attribute of a data column , Only applicable to integer type data columns .

2、 Set up AUTO_INCREMENT The data column of the attribute should be a sequence of positive numbers , Therefore, the data column should be declared as UNSIGNED, In this way, the number of sequences can be doubled .

3、AUTO_INCREMENT Data columns must have unique indexes , To avoid repetition of serial numbers ( That is, the primary key or part of the primary key ).
AUTO_INCREMENT The data column must have NOT NULL attribute .

4、AUTO_INCREMENT The maximum value of the serial number of a data column is constrained by the data type of the column , Such as TINYINT The maximum number of data columns is 127, If you add UNSIGNED, The maximum is 255. Once the ceiling is reached ,AUTO_INCREMENT It will fail. .

5、 When deleting all tables ,MySQL AUTO_INCREMENT From 1 Restart numbering .

This is because when performing a full table operation ,MySQL( and PHP The best combination ) In fact, this optimization operation is done ： First delete all the data and indexes in the data table , Then rebuild the data table .

If you want to delete all data rows and keep the serial number information , You can use a belt like this where Of delete Command to suppress MySQL( and PHP The best combination ) The optimization of the ：delete from table_name where 1;

You can use last_insert_id（） Get the value that has just been self incremented .

explain ： Part of the content comes from search and collation , The purpose is to record the summary , Learn to use ~

summary

This is about MySQL in auto_increment What is the role of the article introduced here , More about MySQL in auto_increment Please search the previous articles of the software development network or continue to browse the relevant articles below. I hope you will support the software development network more in the future ！