how to set dependency on combo-box

i have one query related with fetching data in combo-box.
In my project i have two combox in which 2nd is dependent on first. when i select data from 1st combo bax send need to show respective data only. 
Suppose in 1st when i am selecting teacher 2nd need to show me only teachers
and when i select professor in 1st 2nd only professor.
can you suggest me in which video i will get this 
thank you

You need to make the variable of the 1st combo to be an input parameter of the query for the 2nd combo, you also need an action on change the 1st combo, to redo the query of the 2nd combo.

Hope that helps
thank you for ur reply 
but i know that logic. I want to know how to execute it . Do some videos are available to do this ?
Have you watched the videos availabe here?
And this one in particular?
Hi again, 
Check the following posts maybe they are more explicit:
Post 1
Post 2