PHP/MySQL Interaction

Hi All This is probably off-topic here and my only real excuse is that after having searched google as well as both php and mysql sites/manuals I'm no closer to an answer for something I'd like to get solved this morning... I have two database tables (mysql). One contains information about images (filename, image description, directory path), the other contains two columns - one is the ID of the image and the other is an ID that represents an image category. The reason for doing this is so that I can have an image in multiple categories. Ie. An Image is a product image and also a promotional image (for example). Now I want to randomly select an image from my database such that it belongs in a set category. This isn't a problem and I have the code working fine. I am having trouble with the twist on this though. I want to be able to select an image based on it being in a certain category but also to be able to exclude that image if it is in a different category. That is, I want to select images in category 3 but only if they are not also in category 5 (for example). Can this be done in a single mysql query or am I going to have to link a series of mysql queries together with some php code to firstly get a set of images that are in one category (into an array for eg) and then go through and remove anything that's also in another category and then do a random selection from the remaining contents of the array? I'd like to think it can be done in a single mysql query and it would certainly be more elegant but I haven't been able to figure out a way and I'm starting to think I'll need to do it by manipulating arrays in php which would be far less elegant (although obviously still do-able). Any suggestions anyone? Or references to a tutorial/website somewhere that talks about doing this sort of thing would also be fine. Thanks Dave