Hi all, I'm a little confused with the following concep, and it's hard to explain.

user logs in to the database, and sees user-specific content, such as his profile, no one else's.

That's fine, but what I don't understand is: user/pass/login checks are made against the inherant MYSQL database, obviously, but all my content is going to be in a separate database, the one I'm building. So given there's two, unrelated databases in the equation there, how can I ensure material pulled out is specific to him, since his username in the MYSQL database has no link with his profile in MY database? This isn't easy to make clear, but essentially I don't understand how material pulled out of my database can be directed to certain users if all user info is stored in a different DB, the MYSQL one.

This is of course assuming I've understood the user registration method correctly. For all database-using websites where people sign up, I assume they are registered as users on the MYSQL database?