Leetcode problem: Department Highest Salary - sql

Question

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 | Jim | 90000 | 1 |
| 3 | Henry | 80000 | 2 |
| 4 | Sam | 60000 | 2 |
| 5 | 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, your SQL query should return the following rows (order of rows does not matter).
+------------+----------+--------+
| Department | Employee | Salary |
+------------+----------+--------+
| IT | Max | 90000 |
| IT | Jim | 90000 |
| Sales | Henry | 80000 |
+------------+----------+--------+
My solution is below, Someone please tell me why this is not working
Select Department , Employee ,MAX(Salary) AS SALARY FROM
(select Department.Name AS Department,Employee.Name as Employee, SALARY from employee
LEFT JOIN Department
ON Employee.DepartmentID=Department.ID) as base
GROUP BY Department
The difference between my code output and expected output is given below.
Output
{"headers": ["Department", "Employee", "SALARY"], "values": [["IT", "Joe", 90000], ["Sales", "Henry", 80000]]}
Expected
{"headers": ["Department", "Employee", "Salary"], "values": [["IT", "Jim", 90000], ["Sales", "Henry", 80000], ["IT", "Max", 90000]]}

Answer 1

Someone please tell me why this is not working
Because what you have so far reads like this: "for every department, return an unspecified employee, and also return max salary within this department". Note that max salary is in no way related to the employee that is returned. In this case, it's seems to be returning the first employee for each department. And in case of "Sales", it even turns out to be correct, accidentally.
What you should have written is "for every department, find out its max salary. Then find all employees whose salary is equal to max_salary for their department"

Answer 2

You first need to get the MAX(SALARY) by Department and then query all employee that have this salary in this department.
-- max salary by department, adding DepartmentName to the result
WITH max_salary as (
SELECT Department.DepartmentId, Department.Name as DptName, MAX(SALARY)
FROM Employee JOIN Department
ON Employee.DepartmentID=Department.ID
GROUP BY DepartmentId, Name
)
-- select ALL employees with max salary
SELECT DptName, e.Name, e.SALARY
FROM Employee e JOIN max_salary m
ON e.SALARY=m.SALARY AND e.DepartmentId=m.DepartmentId

Answer 3

Get the highest SALARY for each department, and then return the employees whose salaries are equal to the maximum SALARY. The following query does this.
WITH TB_Salary as (
SELECT MAX(SALARY) AS Salary,DepartmentId
FROM employee
GROUP BY DepartmentId
)
SELECT DISTINCT Department.Name AS Department,employee.Name AS Employee,employee.Salary FROM employee
JOIN TB_Salary ON employee.SALARY = TB_Salary.SALARY AND employee.DepartmentId = TB_Salary.DepartmentId
JOIN Department ON Department.Id = employee.DepartmentId
ORDER BY employee.Salary DESC
OR
SELECT DISTINCT Department.Name AS Department,employee.Name AS Employee,employee.Salary
FROM employee
JOIN
(
SELECT MAX(SALARY) AS Salary,DepartmentId
FROM employee
GROUP BY DepartmentId
) TB_Salary
ON employee.SALARY = TB_Salary.SALARY AND employee.DepartmentId = TB_Salary.DepartmentId
JOIN Department ON Department.Id = employee.DepartmentId
ORDER BY employee.Salary DESC
output: