MySQL_ constraints (cascade operation)

1 Overview

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

• Classification :
  1. Primary key constraint: primary key
  2. Non-empty constraint: not null
  3. Unique constraint: unique
  4. Foreign key constraint: foreign key

2. Non-empty constraints

Non-null constraint : not null, the value of a column cannot be null

• 1. Add constraints when creating a table

CREATE TABLE stu1(
				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. Remove the non-empty constraint of name

ALTER TABLE stu MODIFY NAME VARCHAR(20);

---

3. Unique constraint

unique, the value of a column cannot be repeated

• 1. Note:
  * The unique constraint can have a NULL value, but only one record can be null
• 2. When creating a table, add a unique constraint

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

• 3. Remove the unique constraint

	ALTER TABLE stu DROP INDEX phone_number;

• 4. After the table is created, add a unique constraint

ALTER TABLE stu MODIFY phone_number VARCHAR(20) UNIQUE;

4. Primary key constraint: primary key

• 1. Note:

    	1. 含义：非空且唯一
    	2. 一张表只能有一个字段为主键
    	3. 主键就是表中记录的唯一标识
  

• 2. 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)
			);

• 3. Delete the primary key

			-- 错误 alter table stu modify id int ;
			ALTER TABLE stu DROP PRIMARY KEY;

• 4. After creating the table, add the primary key

ALTER TABLE stu MODIFY id INT PRIMARY KEY;

• 5. Automatic growth:

1. Concept: If a column is of numeric type, use auto_increment to complete the value of automatic growth

2. When creating a table, add primary key constraints and complete primary key self-growth

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

• 3. Delete automatic growth

ALTER TABLE stu MODIFY id INT;

• 4. Add automatic growth

			ALTER TABLE stu MODIFY id INT AUTO_INCREMENT;

5. Foreign key constraints

Foreign key constraint : foreign key, let the table and the table have a relationship, so as to ensure the correctness of the data.

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

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

• 2. Delete foreign keys

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 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
  1. Add cascade operation

语法：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. 级联更新：ON UPDATE CASCADE 
    		2. 级联删除：ON DELETE CASCADE