184. Department Highest Salary

Level: Medium

Question:

Table: Employee

+--------------+---------+
| Column Name  | Type    |
+--------------+---------+
| id           | int     |
| name         | varchar |
| salary       | int     |
| departmentId | int     |
+--------------+---------+
id is the primary key column for this table.
departmentId is a foreign key of the ID from the Department table.
Each row of this table indicates the ID, name, and salary of an employee. 
It also contains the ID of their department.

Table: Department

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| id          | int     |
| name        | varchar |
+-------------+---------+
id is the primary key column for this table.
Each row of this table indicates the ID of a department and its name.

Write an SQL query to find employees who have the highest salary in each of the departments.

Return the result table in any order.

The query result format is in the following example.

Example 1:

Input: 
Employee table:
+----+-------+--------+--------------+
| id | name  | salary | departmentId |
+----+-------+--------+--------------+
| 1  | Joe   | 70000  | 1            |
| 2  | Jim   | 90000  | 1            |
| 3  | Henry | 80000  | 2            |
| 4  | Sam   | 60000  | 2            |
| 5  | Max   | 90000  | 1            |
+----+-------+--------+--------------+
Department table:
+----+-------+
| id | name  |
+----+-------+
| 1  | IT    |
| 2  | Sales |
+----+-------+
Output:
 
+------------+----------+--------+
| Department | Employee | Salary |
+------------+----------+--------+
| IT         | Jim      | 90000  |
| Sales      | Henry    | 80000  |
| IT         | Max      | 90000  |
+------------+----------+--------+
Explanation:
 Max and Jim both have the highest salary in the IT department 
 and Henry has the highest salary in the Sales department.

My Solution:

First, JOIN the Employee table with Department table to find out the max salary in each department. I use a inner select to create a temp table.

Second, find employee(s) that has the max salary in the department.

select t.name as Department, e2.name as Employee, e2.salary
from Employee e2,
(select d.id, d.name, max(salary) as salary
from Employee e join Department d on e.departmentId=d.id
group by d.name) t
where e2.departmentId=t.id and e2.salary=t.salary