MySQL multi table query_ Cartesian product_ Inner connection_ Implicit connection

-- What is multi table query ： Query multiple fields in multiple tables
CREATE DATABASE IF NOT EXISTS mydb_02;
USE mydb_02;
CREATE TABLE dept (
  id INT PRIMARY KEY AUTO_INCREMENT,
  NAME VARCHAR (20)
);
INSERT INTO dept (NAME) 
VALUES
  (' Development Department '),
  (' The Marketing Department '),
  (' Finance Department ') ;
  
SELECT *FROM dept;
DROP TABLE emp;
-- Create an employee table
CREATE TABLE emp (
  id INT PRIMARY KEY AUTO_INCREMENT,
  NAME VARCHAR (10),
  gender CHAR(1),-- Gender  
  salary DOUBLE,-- Wages  
  join_date DATE,-- Date of entry  
  dept_id INT,
  FOREIGN KEY (dept_id) REFERENCES dept (id) -- Foreign keys , Related department table ( The primary key of the Department table ) )
) ;
INSERT INTO emp(NAME,gender,salary,join_date,dept_id) VALUES(' The Monkey King ',' male ',7200,'2013-02-24',1);
INSERT INTO emp(NAME,gender,salary,join_date,dept_id) VALUES(' Pig eight quit ',' male ',3600,'2010-12-02',2);
INSERT INTO emp(NAME,gender,salary,join_date,dept_id) VALUES(' Tang's monk ',' male ',9000,'2008-08-08',2);
INSERT INTO emp(NAME,gender,salary,join_date,dept_id) VALUES(' Bones jing ',' Woman ',5000,'2015-10-07',3);
INSERT INTO emp(NAME,gender,salary,join_date,dept_id) VALUES(' Spider essence ',' Woman ',4500,'2011-03-14',1);

-- Single table query
-- Look up the employee table

SELECT *FROM emp;
SELECT *FROM dept;

-- Multi-table query Query multiple tables
-- demand ： Query all fields in the employee and department tables at the same time
-- A problem ： Is a Cartesian product （A All records in the table *B All records in the table ）--- Field redundancy
SELECT*FROM emp,dept;

-- Solve Cartesian product
-- 1） Which tables to query          The employee table emp, Departmental table dept
-- 2） Query which fields in these tables      Employee list name , Department number , Names of all departments in the Department table
-- 3） The link conditions of these tables      In the list of employees dept_id To associate the primary key of the Department table id

-- Internal connection
-- Implicit inner join Use Basic where sentence Connection condition
-- Display inner connection inner join Connection condition

-- demand : Use implicit inner join queries :   Name in the employee table , department id Check at the same time Department name in the Department table  
-- surface :emp  ,dept
 -- emp name,dept_id
 -- dept name
 -- emp Of dept_id = dept Of id
SELECT 
  emp.`name`,
  emp.`dept_id`,
  dept.`name` 
FROM
  emp,
  dept 
WHERE emp.`dept_id` = dept.`id` ;

-- improvement ： Alias the table , Simplify writing
SELECT 
  t1.`name`,
  t1.`dept_id`,
  t2.`name` 
FROM
  emp t1,
  dept t2 
WHERE t1.`dept_id` = t2.`id` ;