Source: https://leetcode.com/problems/department-highest-salary/#/description
Description:

The Employee table holds all employees. Every employee has an Id, a salary, and there is also a column for the department Id.

+----+-------+--------+--------------+
| Id | Name | Salary | DepartmentId |
+----+-------+--------+--------------+
 

| 1 | Joe | 70000 | 1 |
| 2 | Henry | 80000 | 2 |
| 3 | Sam | 60000 | 2 |
| 4 | Max | 90000 | 1 |
+----+-------+--------+--------------+

The Department table holds all departments of the company.

+----+----------+
| Id | Name |
+----+----------+
| 1 | IT |
| 2 | Sales |
+----+----------+

Write a SQL query to find employees who have the highest salary in each of the departments. For the above tables, Max has the highest salary in the IT department and Henry has the highest salary in the Sales department.

+------------+----------+--------+
| Department | Employee | Salary |
+------------+----------+--------+
| IT | Max | 90000 |
| Sales | Henry | 80000 |
+------------+----------+--------+

Solution:

select  distinct t2.name as department, t1.employee, t1.salary from
(
    SELECT     e1.name as employee, e1.salary as
 
 salary,e1.departmentid  from 
    (selEct * from Employee ) e1
    inner JOIN
    (select departmentid,max(salary) as salary from Employee  group by departmentid) e2
    on e1.departmentid = e2.departmentid and
     e1.salary = e2.salary
)t1
inner JOIN
(select * from Department) t2
on t1.departmentid = t2.id

184. Department Highest Salary (medium)