SQL - Повышение для каждого сотрудника в среднем на зарплату его отдела

Question

Я знаю, как выбрать среднюю зарплату для каждого отдела:

select departments.dept_name, 0.1* round(avg(salary),2) as raise 
from employees 
  inner join departments using (dept_id) 
group by dept_name;

But i have to do this raise for every employee with a result of:

The result i need to achieve

And I can't figure this out

The closest I can get is:

select surname, salary, 
       salary = (case when dept_id is null then 0 else round(avg(salary),2) end) as raise 
from employees 
group by dept_id, surname, salary 
order by surname;

с результатом:

Результат моего кода sql