Interview shock 59: can there be multiple auto increment columns in a table?

Auto incrementing columns can use auto_increment To achieve , When a column is identified as auto_increment after , If you do not set any values for this column when adding , Or set... For this column NULL When the value of , Then it will fill this column with self incrementing rules .

PS： The following content of this article is based on MySQL InnoDB Database engine .

By default, the value of auto increment column is 1, Each increment 1, For example, the following tables are created SQL：

create table tab_incre(
    id int primary key auto_increment,
    name varchar(250) not null
);

When we add , No auto increment id Set any value , Its execution results are as follows ： From the above results, we can see that the default value of auto increment column is 1, Each increment 1.

1. Manually specify self increment

You can manually specify self increment when creating a table , If you do not specify self increment , Then it defaults to 1 As a self increment , Manually specify self increment SQL The order is as follows ：

create table tab_incre(
    id int primary key auto_increment,
    name varchar(250) not null
) auto_increment=50;

Use “show create table table_name” You can view the auto increment column value of the auto increment column in the table , As shown in the figure below ： The self increment of this table is 50, We can also create a piece of data to verify whether the self increment is 50, As shown in the figure below ：

2. Manually modify self increment

After the table is created , We can also pass alter Command to modify the value of the auto increment column , Its modification commands are as follows ：

alter table table_name auto_increment=n;

If you want to tab_incre The self increment in the table is modified as 100, The following can be used: SQL To achieve ：

matters needing attention

When we try to set the self increment to a value smaller than the maximum value in the self increment column , The self increment will automatically become the maximum value of the self increment column +1 Value , As shown in the figure below ：

3. Can a table have multiple auto increment Columns ？

There can only be one self incrementing column in a table , This is similar to the rule that a table can only have one primary key , When we try to add a self incrementing column to a table , It can be added successfully , As shown in the figure below ： When we try to add multiple auto incrementing columns to a table , You will be prompted that there can only be one self incrementing error message , As shown in the figure below ：

4. Other matters needing attention

Except that only one auto increment column can be added to a table , The following two issues need to be paid attention to in auto incrementing .

4.1 Auto incrementing column can only be of integer type

The field type of auto increment column can only be integer type （TINYINT、SMALLINT、INT、BIGINT etc. ）, As shown in the figure below ： When we use other types as data types for auto incrementing Columns , The following error will be prompted ：

4.2 Must cooperate key Use it together

auto_increment Must cooperate key Use it together , This key It can be primary key or foreign key, without key You're going to report a mistake , As shown below ：

PS：auto_increment You can also work with unique constraints unique Use it together .

summary

The default value of auto increment column is 1, Each increment 1, However, you can also manually specify self increment when creating tables , Of course, in special cases, after the table is created , It can also be done through alter Modify self increment . There can only be one self incrementing column in a table , Just like a table can only have one primary key , If you set multiple auto increment Columns , that SQL Execution will report an error . In addition, note that the auto increment column should be of integer type , And auto_increment Need to cooperate with key Use it together , This key It can be primary key or foreign key.

It's up to you to judge right and wrong , Disdain is to listen to people , Gain or loss is more important than number .

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