getting session user info on custom php page

If I create a new page in my drupal system with mostly php code in it, can I and how I use the global $user variable to retrieve the username (field "name" in the users table) ?
The very username of the person connected (so it's in the session or something ?) so when he goes on that page i'll be able to show him a list of clients he's in charge of , this list coming from an external DB that can link his username with it (actually an oracle view).

and what would be the most efficient/elegant way to have this other external oracle DB connection in my drupal page.

Thanks a lot.