Hi, all of my teachers !

In a oracle SQL study guide it says " Do not use order by clause in subquery". But as you see this code snippet below

create view exam
as
select ename,sal
from emp
order by dept

This code I wrote didn't cause error at all ! Then , now I wonder why the text book recommend not to use order by clause in a subquery in a view.

Can anyone explain me why?

Thank in advance .