Simple SQL query returning null value

I just started studying SQL queries. I am practicing on this site: https://www.techonthenet.com/sql/joins_try_sql.php

I want to find:

“the name of the employee with the highest salary for every department”.

My query is:

SELECT first_name, max(salary) FROM employees, departments 
WHERE departments.dept_id=employees.dept_id
GROUP BY employees.dept_id

And I get null value for first_name : I understand that the problem is due to the group by expressions. But how can I solve this?

Tables:

Answer

You can alternatively use row_number() like below, you don’t need to join to departments table unless you need to show the name of the department:

Select e.*
from employees e
INNER JOIN
(
   SELECT e.id, e.dept_id. e.first_name, 
          rn=row_number() over (partition by e.dept_id order by e.salary desc)
   FROM employees e 
) x ON x.id = e.id
where x.rn = 1

EDIT

(Since OP does not want to use row_number() function amd it turned out the query will be used in mysql instead of sql server) -> Can you please try this:

select em.*
from employees em, (
    Select dept_id, max(salary) salary
    from employees e
    group by dept_id
) x on x.dept_id=em.dept_id and x.salary = em.salary

That should work but the online compiler does not accept join with a sub-query as far as I understand. Easiest solution I can think of, in this case:

select em.*
from employees em
where salary = (select max(salary) from employees em2 where em.dept_id = em2.dept_id)