①. Overview of subqueries

• ①. Subquery refers to a query that is nested within another query statement by one query statement , This feature comes from MySQL 4.1 Start introducing

• ②. Subquery （ Internal query ） In the main query ( External query ) The previous execution completed

• ③. The results of the sub query are by the main query （ External query ） Use

• ④. Sub query considerations :

1. Subqueries should be enclosed in parentheses
2. Place the subquery to the right of the comparison criteria
3. Single line operators correspond to single line subqueries , Multiline operators correspond to multiline subqueries

#  Whose pay ratio is Abel high 
# Mode one ： 
SELECT salary FROM employees WHERE last_name = 'Abel'; 
SELECT last_name,salary FROM employees WHERE salary > 11000;
# Mode two ： Self join  
SELECT e2.last_name,e2.salary FROM employees e1,employees e2 
WHERE 
e1.last_name = 'Abel' AND e1.`salary` < e2.`salary`
# Mode three ： Subquery  
SELECT last_name,salary FROM employees 
WHERE 
salary > ( SELECT salary FROM employees WHERE last_name = 'Abel' );

• ⑤. We return one or more records according to the results of internal query , Sub query is divided into single line sub query 、 Multi line sub query
  
• ⑥. We check whether the query is executed more than once , Divide the subquery into relevant ( Or associated ) Subquery and unrelated ( Or unrelated ) Subquery

1. The subquery queries the data results from the data table , If this data result is executed only once , The data result is then executed as a condition of the main query , Then such a subquery is called an unrelated subquery
2. If the subquery needs to be executed multiple times , That is, in the form of circulation , Start with an external query , Every time a subquery is passed in to query , The results are then fed back to the outside world , This nested execution is called related subquery

②. Subquery - Single line sub query

• ①. Single line comparison operator
  

• ②. Subquery title - sql Exhibition

# (1). return job_id And 141 Same as employee No ,salary Than 143 The name of the employee with a large number of employees ,job_id And wages 
SELECT
	last_name,
	job_id,
	salary 
FROM
	employees 
WHERE
	job_id = ( SELECT job_id FROM employees WHERE employee_id = 141 ) 
	AND salary > ( SELECT salary FROM employees WHERE employee_id = 143 );
# (2). Return to the lowest paid employees of the company last_name,job_id and salary
SELECT
	last_name,
	job_id,
	salary 
FROM
	employees 
WHERE
	salary = ( SELECT MIN( salary ) FROM employees );
# (3). Query and 141 Number or 174 Of employee No manager_id and department_id Of the same other employees employee_id, manager_id,department_id
SELECT
	employee_id,
	manager_id,
	department_id 
FROM
	employees 
WHERE
	manager_id IN ( SELECT manager_id FROM employees WHERE employee_id IN ( 174, 141 ) ) 
	AND department_id IN ( SELECT department_id FROM employees WHERE employee_id IN ( 174, 141 ) ) 
	AND employee_id NOT IN ( 174, 141 )
# (4). The minimum wage is greater than 50 The Department of minimum wage id And its minimum wage 
SELECT
	department_id,
	MIN( salary ) 
FROM
	employees 
GROUP BY
	department_id 
HAVING
	MIN( salary ) > ( SELECT MIN( salary ) FROM employees WHERE department_id = 50 );
# (5). Explicit employee employee_id,last_name and location. among , If employee department_id And location_id by 1800  Of department_id identical , be location by ’Canada’, The rest are ’USA 
SELECT
	employee_id,
	last_name,
	( CASE department_id WHEN ( SELECT department_id FROM departments WHERE location_id = 1800 ) THEN 'Canada' ELSE 'USA' END ) location 
FROM
	employees;