correlated subquries

I am reading the OCP preperation book and it states there that subquries are bad and alternate methods should be used.

There is the subquery mentioned in the book but no alternate solution .  can someone point out an alternate solution for this?

the following query list all employees whose salary is less than the average salary of their department.

select p.last_name, p.department_id from employees p
where p.salary < ( select avg(s.salary) from employees s
where s.department_id=p.department_id)
/