Another solution for AVG salaries

you can also use Right Join with the dept_manager because we need all the managers anyway

Code:

SELECT

  d.dept_name,  avg(salary) as average_salary

FROM  

      departments d

         right JOIN  

    dept_manager dm  on d.dept_no = dm.dept_no
    
    join 
     salaries s on s.emp_no = dm.emp_no 
     
    group by d.dept_name
    order by average_salary desc
;