We spice the SQL statement a bit add the requirement to show in a dedicated column the gap between max and min salary, displaying as column name a short label, and ordering the report accordingly to the gap:
select department_id as dep, max(salary) - min(salary) as gap,
max(salary) as max, min(salary) as min
group by department_id
order by gap desc;
A fun basic book on SQL: Head First SQL.