MySQL foundation - constraints

Catalog

constraint

Case study

Foreign key constraints

Add foreign keys

Delete foreign key

Behavior

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In the last article, we learned MySQL Basics —— function , This article is for us to learn MySQL Basics —— constraint .

constraint

Constraints are rules that act on fields in a table , Used to limit the data stored in a table , Its purpose is to ensure that the data in the database is correct 、 Effectiveness and integrity .

Common constraints are as follows ：

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	PRIMAPY KEY
Default constraint	When saving data , When the value of this field is not specified , The default value is used	DEFAULT
Detection constraints (8.0 After the version )	Ensure that the field value meets a certain condition	CHECK
Foreign key constraints	Make a connection between the data of two tables , Ensure data consistency and integrity	FOREIGN KEY

Be careful ： Constraints act on fields in a table , You can create a table / Add constraints when modifying tables .

Case study

Next, we use a case to demonstrate how to set constraints when creating a data table .

Case requirements are as follows ：

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 unique	NOT NULL,UNIQUE
passwrod	password	varchar(10)	The default value is 123456	DEFAULT
gender	Gender	char(1)	nothing

The sample code is as follows ：

CREATE TABLE students(
    id int PRIMARY KEY AUTO_INCREMENT COMMENT ' user id',
    name varchar(10) NOT NULL UNIQUE COMMENT ' full name ',
    password varchar(10) DEFAULT '123456' COMMENT ' password ',
    gender char(1) COMMENT ' Gender '
)COMMENT ' Student list ';

As shown in the figure below ：

This will successfully create constraints for fields when creating tables .

Foreign key constraints

Foreign keys are used to connect the data of two tables , So as to ensure the consistency and integrity of data .

Add foreign keys

Add foreign key syntax format as follows ：

# Add foreign keys when creating tables 
CREATE TABLE  Table name (
     Field name   data type ,
    ...
    [CONSTRAINT] [ Foreign key name ] FOREIGN KEY ( Foreign key field name ) REFERENCES  Main table ( Name of main table )
);
​
# Add a foreign key after creating a table 
ALTER TABLE  Table name  ADD CONSTRAINT  Foreign key name  FOREIGN KEY ( Foreign key field name ) REFERENCES  Main table ( Name of main table );

The sample code is as follows ：

#  Word list 
CREATE TABLE classinfo(
    id int PRIMARY KEY AUTO_INCREMENT COMMENT ' class id',
    name varchar(50) NOT NULL COMMENT ' Class name '
)COMMENT ' Class table ';
​
​
#  Main table 
CREATE TABLE students(
    id int PRIMARY KEY AUTO_INCREMENT COMMENT 'id',
    name varchar(50) NOT NULL COMMENT ' Student name ',
    password varchar(10) DEFAULT '123456' COMMENT ' password ',
    gender char(1) COMMENT ' Gender ',
    classid int NOT NULL COMMENT ' class id',
    CONSTRAINT fk_students_classinfo_id FOREIGN KEY (classid) REFERENCES classinfo(id)
)COMMENT ' Class table ';

As shown in the figure below ：

 

Delete foreign key

The syntax format for deleting foreign keys is as follows ：

ALTER TABLE  Table name  DROP FOREIGN KEY  Foreign key name ;

The sample code is as follows ：

ALTER TABLE students DROP FOREIGN KEY fk_students_classinfo_id;

As shown in the figure below ：

Next, we add foreign keys in another way , The code is as follows ：

ALTER TABLE students ADD CONSTRAINT fk_student_classinfo FOREIGN KEY (classid) REFERENCES classinfo(id);

As shown in the figure below ：

Behavior

After setting the foreign key , You can't easily delete the data in the data table , This is because there is a foreign key constraint behavior —— Delete / Update behavior .

Foreign key deletion / Update behaviors are ：

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 .
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 .
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（ Foreign keys are required and allowed to be 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 ）

Set delete for foreign keys / The update behavior syntax format is as follows ：

ALTER TABLE  Table name  ADD CONSTRAINT  Foreign key name  FOREIGN KEY ( Foreign key field ) REFERENCES  Main table name ( Main table fields ) ON UPDATE  Behavior  ON DELETE  Behavior ;

The sample code is as follows ：

ALTER TABLE students ADD CONSTRAINT fk_student_classinfo FOREIGN KEY (classid) REFERENCES classinfo(id) ON UPDATE CASCADE ON DELETE CASCADE;

As shown in the figure below ：

Be careful ： Delete / The update behavior should be consistent .

Okay ,MySQL Basics —— That's where constraints come in , Next article study MySQL Basics —— Multi-table query .