Pulling an ID list from a DB, then filtering that list against another list of ID's

Hey All- Got an interesting problem! I have a list of ID's, basically things people have viewed on my website. When I pull the viewable items from my database, I need to be able to filter out all the ID's people have already viewed, so that they only get a list of things they have not viewed. I can think of a way to do this, but it is a nasty loop situation, that would probably not be good and tie up significant CPU time. Anyone have any suggestions on ways to filter view items from a fresh list that I am pulling from my Database? RH9, PHP 4.3.2, MySQL 4.0.14-standard, Apache 2.0.47 Thanks! Matt