PL/SQL :: Using A Subquery To Display Names And Max Salaries By Department / HR Schema

Using the HR schema, I'm trying to write a query which will display the names and salaries of those employees who earn the highest salaries in their respective departments.

I wrote the query:

SELECT first_name, last_name, salary
FROM employees
WHERE salary =
(SELECT department_id, MAX(salary) FROM employees GROUP BY department_id);

However, this query does not execute correctly but I'm not sure why?