Default constraint and zero fill constraint of MySQL constraint

Catalog

Default constraint

Add default constraint

Delete default constraint

Zero fill constraint

  summary

Default constraint

MySQL The default value constraint is used to specify the default value of a column .

Add default constraint

The way 1： < Field name > < data class type > default < Default value >;

The way 2: alter table  Table name  modify  Name   class type  default  Default value ;

create table t_user10 (id int ,name varchar(20) ,address varchar(20) default ‘ Beijing ' -- Specify the default constraint );-- alter table Table name modify Name type default The default value is ;create table t_user11 (id int ,name varchar(20) ,address varchar(20));alter table t_user11 modify address varchar(20) default ‘ Beijing '; Delete default constraint -- alter table < Table name > modify column < Field name > < type > default null;alter table t_user11 modify column address varchar(20) default null; Zero fill constraint

1、 When inserting data , When the length of the value of this field is less than the defined length , This value will be preceded by the corresponding 0
2、zerofill The default is int(10)
3、 When using zerofill when , By default, it will automatically add unsigned（ Unsigned ） attribute , Use unsigned After attribute , The numerical range is of the original value 2 times , for example , The sign is -128~+127, No sign is 0~256.

such as ： Now set the length of a field to 5, So the real data is 1, So what appears in your database is 00001;

create table t_user12 (id int zerofill , -- Zero fill constraint name varchar(20));alter table t_user12 modify id int;-- 1. Adding constraints create table t_user12 (id int zerofill , -- Zero fill constraint name varchar(20));insert into t_user12 values(123, ' Zhang San ');insert into t_user12 values(1, ' Li Si ');insert into t_user12 values(2, ' Wang Wu ');

  summary

We learned a lot of constraints earlier , Here is a general summary , We need to set these constraints according to our own application scenarios , Finally, we can meet our daily needs ：

-- summary -- 1： Add a primary key constraint by modifying the table structure create table emp4(eid int primary key,name varchar(20),deptId int,salary double);-- 2: Add a self increasing positive constraint create table t_user1 (id int primary key auto_increment,name varchar(20));-- 3： Create a non empty constraint create table mydb1.t_user6 (id int ,name varchar(20) not null, -- Specify a non NULL constraint address varchar(20) not null -- Specify a non NULL constraint );-- 4： Create a unique constraint create table t_user8 (id int ,name varchar(20) ,phone_number varchar(20) unique -- Specify a unique constraint );-- 5: Create default constraints create table t_user10 (id int ,name varchar(20) ,address varchar(20) default ' Beijing ' -- Specify the default constraint );

This is about MySQL Default constraint of constraint default And zero fill constraints zerofill This is the end of the article , More about MySQL constraint Please search the previous articles of software development network or continue to browse the relevant articles below. I hope you will support software development network more in the future ！