MySQL constraint_ Foreign key constraint

-- Foreign key constraints （foreign key）
-- Create an employee table employee
-- Employee number id, Name of employee name, Department name dept_name, Address of the employee address
CREATE TABLE employee(
      id INT PRIMARY KEY AUTO_INCREMENT,
      NAME VARCHAR(10),  
      address VARCHAR(20),
      dept_name VARCHAR(20)
);
-- Insert data into the employee table
INSERT INTO employee (NAME,address,dept_name)
   VALUES(' article ',' Beijing ',' R & D department '),
         (' Ke faxing ',' Xi'an ',' Testing department '),
         (' Round and round ',' Shanghai ',' R & D department '),
         (' Zhang San ',' Shenzhen ',' Implementation Department '),
         (' Li Si ',' Guangzhou ',' Implementation Department '),
         (' Zhao Youting ',' Hong Kong ',' Testing department ');
-- Query all data in the employee table
SELECT * FROM employee ;
DROP TABLE employee;

-- Inquire about Employee's name and department name
SELECT 
  NAME ' full name ',
  dept_name ' Department name '
FROM
  employee ;
/*
     problem :
        id = 1         R & D department
        id = 3         R & D department
        
        id = 2         Testing department
        id = 6         Testing department
        
        ...
        
         Field redundancy ( Too much repetition ), The queried data is not very intuitive
*/
-- Improve the above problem ： Split the employee table into two tables
-- The employee table ： Record employee information
-- Departmental table ： Record the number and name of the Department
DROP TABLE employee;

-- Departmental table ：dept surface ： The primary key grows by itself id( Department number )  dept_name Department name
CREATE TABLE dept (
  id INT PRIMARY KEY AUTO_INCREMENT,
  dept_name VARCHAR (10)
) ;
DROP TABLE dept;
-- insert data
INSERT INTO dept (dept_name) 
VALUES
  (' R & D department '),
  (' Testing department '),
  (' Implementation Department ') ;
SELECT * FROM dept;

-- Create an employee table
CREATE TABLE employee(
     id INT PRIMARY KEY AUTO_INCREMENT, -- Employee number
     NAME VARBINARY(10), -- Employee name
     address VARBINARY(20), -- Employee address
     dept_id  INT -- Department number
);
INSERT INTO employee (NAME,address,dept_id)
   VALUES(' article ',' Beijing ',1),
         (' Ke faxing ',' Xi'an ',2),
         (' Round and round ',' Shanghai ',1),
         (' Zhang San ',' Shenzhen ',3),
         (' Li Si ',' Guangzhou ',3),
         (' Zhao Youting ',' Hong Kong ',2);

SELECT *FROM employee;

SELECT 
  NAME ' full name ',
  dept_id ' Department number '
FROM
  employee ;

-- Continue to improve ： To find a way, you need to set the department number primary key in the Department table id And department table of employee table dept_id The relationship between

-- Solution ： Add foreign key
-- Main table （ Departmental table :dept）, From the table employee( The table where the foreign key is located )
-- constraint Statement Name of the foreign key （ Main table name _ From table name _fk）
    -- foreign key ( From the fields of the table ) reference The name of the main table （ Primary key name ）;
DROP TABLE employee;    
-- Create an employee table and add foreign key constraints
CREATE TABLE employee(
   id INT PRIMARY KEY AUTO_INCREMENT,-- Employee number
    NAME VARBINARY(10), -- Employee name
    address VARCHAR(20),-- Employee address
    dept_id INT,
    CONSTRAINT -- Statement
    dept_emp_fk FOREIGN KEY(dept_id)
    REFERENCES dept(id)
);

-- Add data to the employee table
INSERT INTO employee (NAME,address,dept_id)
   VALUES(' article ',' Beijing ','1'),
         (' Ke faxing ',' Xi'an ','2'),
         (' Round and round ',' Shanghai ','1'),
         (' Zhang San ',' Shenzhen ','3'),
         (' Li Si ',' Guangzhou ','3'),
         (' Zhao Youting ',' Hong Kong ','2');

SELECT *FROM dept;
SELECT*FROM employee;
-- Add data ( Department number must exist , Otherwise, the report will be wrong !)
INSERT INTO employee (NAME,address,dept_id) VALUES (' Ma Yili ',' Xi'an ','4');
-- Due to saving foreign keys , You cannot directly add or modify slave table data ( Related to fields declared by foreign keys )
-- First, operate the main table ----> Then you can operate the slave table
-- Add operation
-- Main table
INSERT INTO dept(dept_name)VALUES(' The sales department ');

-- Delete foreign key
-- alter table From table name DROP FOREIGN KEY Name of the foreign key
ALTER TABLE employee DROP FOREIGN KEY dept_emp_fk;

-- adopt sql Statement to add foreign keys
ALTER TABLE employee
    ADD CONSTRAINT -- Statement
        dept_emp_fk FOREIGN KEY(dept_id)
        REFERENCES dept(id);