Talk about database constraints

constraint

• Concept: The data in the table are defined to ensure the accuracy, effectiveness and integrity of data.
• Category: ① primary key constraint:. Primary key ② non-null constraint:. Not null③ unique constraints:.. Unique ④ foreign key constraint: foreign key
• Non-empty constraint : not null, the value can not be null

--创建表时添加约束
CREATE TABLE stu(
		id INT,
		NAME VARCHAR(20) NOT NULL
   );
--创建表完后，添加非空约束
ALTER TABLE stu MODIFY NAME VARCHAR(20) NOT NULL;
--删除name的非空约束
ALTER TABLE stu MODIFY NAME VARCHAR(20);

• The only constraint : unique, value can not be repeated

--创建表时，添加唯一约束
CREATE TABLE stu(
		id INT,
		phone_number VARCHAR(20) UNIQUE -- 添加了唯一约束			
);
--注意mysql中，唯一约束限定的列的值可以有多个null
				
--删除唯一约束
ALTER TABLE stu DROP INDEX phone_number;
		
--在创建表后，添加唯一约束
ALTER TABLE stu MODIFY phone_number VARCHAR(20) UNIQUE;

• Primary key constraint : Primary Key
  1) When you create 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) Drop Primary: ALTER TABLE stu DROP PRIMARY KEY;
3) After creating the table, add the primary key: ALTER TABLE stu MODIFY id INT PRIMARY KEY;
4) automatic growth: If the column is a numeric type, can be done automatically using auto_increment growth

--在创建表时，添加主键约束，并且完成主键自增长
create table stu(
	id int primary key auto_increment,-- 给id添加主键约束
	name varchar(20)
);
--删除自动增长
ALTER TABLE stu MODIFY id INT;
-- 添加自动增长
ALTER TABLE stu MODIFY id INT AUTO_INCREMENT;

• Note:.. ① a table can have only one primary key field ② primary key is the unique identification of records in the table
• Foreign key constraint : foreign key, so that the table to produce table relationship, so as to ensure accuracy of the data
• When you create a table, you can add foreign key syntax is as follows:

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)
);

• Remove the foreign key:ALTER TABLE 表名 DROP FOREIGN KEY 外键名称;
• After creating the table, add 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);

• Cascade operation:

--添加级联：
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  ;
--级联更新：
ON UPDATE CASCADE
-- 级联删除
ON DELETE CASCADE