Several methods of the minimum value in the maximum value of group query

Several methods of grouping and querying the minimum value of the maximum value

give an example ： Query the lowest department in the highest salary of each department

This question is a bit tongue twisty , Breaking it down is to find out the maximum salary of each department , Then find out which department has the lowest salary

This involves sub queries , There are several ways to do this

The way 1：

You can find out the maximum salary of each department , Then sort in positive order , Take the first one

# The way 1
SELECT MAX(salary)
FROM employees
GROUP BY department_id
ORDER BY MAX(salary)
LIMIT 1;

The way 2：

You can first calculate the maximum salary of each department , Then find the smallest value

# The way 2
SELECT MIN(max_sal)
FROM (
	SELECT MAX(salary) max_sal
	FROM employees
	GROUP BY department_id
) t

The way 3：

Find out the maximum salary of each department , Get any result less than or equal to the result set according to the result

# The way 3
SELECT department_id, MAX(salary)
FROM employees 
GROUP BY department_id
HAVING MAX(salary)<=ALL(	--  All values less than the result ,ANY For any one of 
	SELECT MAX(salary) max_sal
	FROM employees
	GROUP BY department_id
)