Sub query of multi table query_ Single row and single column

-- Sub query of multi table query
-- Single row single row --> Using operators ：<=...
-- Multi row single row -- Combine in( value 1, value 2, value 3)
-- Multiple rows and columns -- Put the data queried in a table --- As a virtual table

-- demand : Query the information of the employee with the highest salary !
SELECT 
  * 
FROM
  emp 
WHERE salary = 
  (SELECT 
    MAX(salary) 
  FROM
    emp) ;

-- demand : Query the information of employees whose salary is less than the average salary  -- Using operators <,>=.=,<=....
SELECT 
  * 
FROM
  emp 
WHERE salary < 
  (SELECT 
    AVG(salary) 
  FROM
    emp) ;

SELECT *FROM emp;
 

-- demand : Query the employee information in the finance department or the marketing department
SELECT 
  * 
FROM
  emp 
WHERE emp.`dept_id` IN 
  (SELECT 
    dept.`id` 
  FROM
    dept 
  WHERE NAME = ' The Marketing Department ' 
    OR NAME = ' Finance Department ') ;
 

-- The third case of subquery : Multiple rows and columns
-- Query employees whose entry date is later than '2011-11-11' Then the employee information and department name

SELECT 
  t1.*,
  -- All fields in the virtual table
  t2.name -- Department table Department name
FROM
  (SELECT 
    * 
  FROM
    emp 
  WHERE join_date > '2011-11-11') t1 -- Virtual table
  LEFT JOIN dept t2 
    ON t1.dept_id = t2.id ;

-- Common links
SELECT 
  t1.*,
  t2.name 
FROM
  emp t1,
  dept t2 
WHERE t1.join_date > '2011-11-11' 
  AND t1.dept_id = t2.id ;