MySQL constraint non-empty constraint, unique constraint, primary key constraint, foreign key constraint, default constraint, check constraint

MySQL constraints

Concept: Limit the data in the table to ensure the correctness, validity and completeness of the data .

classification:

1. Non-empty constraint: not null
2. Unique constraint: unique
3. Primary key constraint: primary key
4. Foreign key constraints: foreign key
5. Default constraint: Default
6. Check constraints: Check

Non-empty constraint: not null

The value of a column cannot be empty

1. Add constraints when creating a table

   CREATE TABLE stu(
   	id INT,
   	NAME VARCHAR(20) NOT NULL -- name为非空
   );
   

2. After creating the table, add a non-empty constraint

	ALTER TABLE stu MODIFY NAME VARCHAR(20) NOT NULL;

3. Delete the name of non-empty constraint

ALTER TABLE stu MODIFY NAME VARCHAR(20); 

The only constraint: unique

The value of a column cannot be repeated

note:

• The unique constraint can have a NULL value, but only one record can be null

1. When creating a table, add a unique constraint

   CREATE TABLE stu(
   	id INT,
   	phone_number VARCHAR(20) UNIQUE -- 手机号
   );
   

2. After the table is created, add a unique constraint

   ALTER TABLE stu MODIFY phone_number VARCHAR(20) UNIQUE;
   

3. Delete unique constraints (different from non-empty constraints)

   ALTER TABLE stu DROP INDEX phone_number;
   

Primary key constraint: primary key

Non-empty and unique

Note:
2. A table can only have one field as the primary key
3. The primary key is the unique identifier of the records in the table

1. When creating a table, add a primary key constraint

   create table stu(
   	id int primary key,-- 给id添加主键约束
    name varchar(20)
   );
   

2. After creating the table, add the primary key

   ALTER TABLE stu MODIFY id INT PRIMARY KEY;
   

3. Delete primary key

   ALTER TABLE stu DROP PRIMARY KEY;
   

4. Automatic growth：

**Note: **Auto growth is based on the value of the previous data. The data set for automatic growth can also be assigned actively.

• Concept: If a column is numeric type , the use auto_incrementcan be done automatically worth growth (and typically along with primary key)

• When creating a table, add a primary key constraint, and complete the primary key self-growth

  create table stu(
  id int primary key auto_increment,-- 给id添加主键约束
  name varchar(20)
  );
  

• Add automatic growth

  ALTER TABLE stu MODIFY id INT AUTO_INCREMENT;
  

• Delete automatic growth

  ALTER TABLE stu MODIFY id INT;
  

​

Foreign key constraint: foreign key

Let the table have a relationship with the table to ensure the correctness of the data.

1. When creating a table, you can add foreign keys

create table 表名(
	....
	外键列
	constraint 约束（外键）名称 foreign key (外键列名称) references 主表名称(主表列名称)
);

2. Delete foreign key

ALTER TABLE 表名 DROP FOREIGN KEY 外键名称;

3. After creating the table, add the foreign key

ALTER TABLE 表名 ADD CONSTRAINT 约束（外键）名称 FOREIGN KEY (外键字段名称) REFERENCES 主表名称(主表列名称);

4. Cascade operation
5. Add cascade operation

  ALTER TABLE 表名 ADD CONSTRAINT 外键名称 
  					FOREIGN KEY (外键字段名称) REFERENCES 主表名称(主表列名称) 
  									ON UPDATE CASCADE ON DELETE CASCADE  ;

2. Classification:
   1. Cascade update: ON UPDATE CASCADE
   2. Cascade delete: ON DELETE CASCADE(use with caution )

Default constraint: Default

Set the default value of the attribute, when it is not defined, the default value is null

1. When creating a table, you can add default constraints

   CREATE TABLE Persons
   (
   Id int NOT NULL,
   Name varchar(255)  DEFAULT '张三'
   );
   

2. After creating the table, add default constraints

   ALTER TABLE Persons
   ALTER Name SET DEFAULT '张三';
   

3. Remove default constraints

   ALTER TABLE Persons
   ALTER Name DROP DEFAULT
   

Check constraint: Check

Used to limit the range of values ​​in the column

1. When creating a table, add check constraints

   Set constraint name

   CREATE TABLE Persons
   (
   Id int NOT NULL,
   Name varchar(255) NOT NULL,
   Address varchar(255),
   City varchar(255),
   CONSTRAINT chk_Person CHECK (Id>0 AND City='北京')
   );
   

   Do not set constraint name

   CREATE TABLE Persons
   (
   Id int NOT NULL,
   Name varchar(255) NOT NULL,
   Address varchar(255),
   City varchar(255),
   CHECK (Id>0)
   );
   

2. After creating the table, add check constraints

   Set constraint name

   ALTER TABLE Persons
   ADD CONSTRAINT chk_Person CHECK (Id>0 AND City='北京');
   

   Do not set constraint name

   ALTER TABLE Persons
   ADD CHECK (Id>0);
   

3. Delete check constraint

   ALTER TABLE Persons DROP CHECK chk_Person;