Simple SQL for category image update

Hi. I need some very simple sql creating for my oscommerce website. I have many categories and need a lot of them updating to include a category image. I have set a default category image of [login to view URL], but I have manually updated some categories but I'm finding this is taking forever! I want some sql to run that would look at each category and if the image for that category was [login to view URL] or blank then find the first image from the first product located in that category/sub category and use that image for the cateory image. Hope I've made this clear. I rekon this should be a two minute job for an experienced sql programmer. Thanks