Two SelectLists the second one dependent to the first one

Hi

I need your support with a problem I faced in APEX.

 I have a form with 2 SelectLists:

• The first one P2_DB contains name of my databases and the source comes from simple query

select d.dbid, d.dbid from pt_database where ....

It works and all expected data are displayed correctly.

• The second one P2_USERNAME, should list all logins/usernames belongs to database pointed in the first SelectList. It is kind of slave to the first SelectList. And now my problem starts. thought i was enough to add as a source similiar query , something like :

SELECT dbuser, username FROM PT_USER_PERMISSION  where dbid=(:P2_DB)