MySQL constraint

constraint
Rules that act on fields in a table , Users restrict data stored in tables
Purpose : Ensure that the data in the database is correct , Effectiveness and integrity .
Constraints act on fields in a table , You can create a table / Add constraints when modifying tables .
classification :

constraint	describe	keyword
Non empty constraint	Limit the data of this field to null	NOT NULL
Unique constraint	Ensure that all data in this field is unique , Not repeated	UNIQUE
Primary key constraint	A primary key is the unique identifier of a row of data , It is required to be non empty and unique	PRIMARY KEY
Default constraint	When saving data , If the value of this field is not specified , The default value	DEFAULT
Check constraint (8.016 After the version )	Ensure that the field value meets a certain condition	CHECK
Foreign key constraints	Used to establish a connection between the data of two tables , Ensure data consistency and integrity	FOREIGN KEY

Case study : According to the demand , Complete the creation of table structure

Field name	Field meaning	Field type	constraint condition	Constraint keywords
id	ID Unique identification	int	Primary key , And grow automatically	PRIMARY KEY, AUTO_INCREMENT
name	full name	varchar(10)	Not empty , And only	NOT NULL,UNIQUE
age	Age	int	Greater than 0, And less than or equal to 120	CHECK
status	state	char(1)	If this value is not specified , The default is 1	DEFAULT
gender	Gender	char(1)	nothing

create table user(
    id int primary key auto_increment comment 'ID Unique identification ',
    name varchar(10) not null unique comment ' full name ',
    age int check (age>0 && age<=120) comment ' Age ',
    status char(1) default '1' comment ' state ',
    gender char(1) comment ' Gender '
)comment ' User table ';

Foreign key constraints
Make a connection between the data of two tables , Ensure data consistency and integrity
Add foreign keys の grammar :

Create table  Table name (
		 Field name   data type ,
		...
		[Constraint] [ Name of the foreign key ] Foreign Key( Foreign key field name ) References  Main table ( Name of main table )
);
or
Alter Table  Table name  Add Constraint  Name of the foreign key  Foreign Key( Foreign key field name ) References  Main table ( Name of main table );

Delete foreign key の grammar

Alter Table  Table name  Drop Foreign Key  Name of the foreign key ;

Foreign key deletion / Update behavior

Behavior	explain
NO ACTION	When deleting in the parent table / When updating the corresponding record , First, check whether the record has a corresponding foreign key , If there is, it is not allowed to delete / to update .( And RESTRICT Agreement )
RESTRICT	When deleting in the parent table / When updating the corresponding record , First, check whether the record has a corresponding foreign key , If there is, it is not allowed to delete / to update .( And NO ACTION Agreement )
CASCADE	When deleting in the parent table / When updating the corresponding record , First, check whether the record has a corresponding foreign key , If there is , Then delete / Update the record of the foreign key in the sub table
SET NULL	When deleting the corresponding record in the parent table , First, check whether the record has a corresponding foreign key , If yes, set the foreign key value in the sub table to null( This requires that the foreign key be allowed to take null)
SET DEFAULT	When the parent table changes , The sub table sets the foreign key column to a default value (Innodb I won't support it )