Restrict screens and data access to users' own information

[nHx4U]

I'm new to all this. I have a database and a number of user screens. I want to control which screens/forms a user sees - as well as restrict him to just generic data (inventory) and only view and edit his own purchase orders. Do I need to add username and password fields to all screens and database tables and then create a filter based on who is logged in? Or is there a simpler EASIER way - built-in to Qt?

Thanks.