The way I designed it so far, the users actually don't need to know the structure of the database. I present them with a web form that contains select boxes for tables, fields, operators and even sometimes values. The table names are comprehensive enough so that someone having the right vocabulary can zoom in on a field in very little time and write a filter (ad-hoc query) fairly quickly. I think this is a strong point of my application and I want to hold on to it.

I'll send you a private message with some info on how to connect. It'll be easier if I show you...