Sorry, I didn't understand your question as clearly as Zaxo. htDig's relational database integration capabilities are pretty much nil, AFAIK.

Still, it might be easier to index the end-product - the rendered pages - using an existing product, rather than the database itself and XML/txt/HTML sources in a roll-your-own system. Then you wouldn't have to worry about reconstructing the URLs from the search results, and since you've already got category->url mapping, you can build a user search interface that allows limiting by categories by allowing search restriction by URL path.

As an aside, it's also quite hard to do good free-text searches within an RDBMS - MySQL's FREETEXT indexes are pretty limited. On the site I'm working on at the moment, we've ditched a search system build round Oracle's ConText / Intermedia search tool in favour of an htDig system indexing the rendered pages within a CMS.

cheers
ViceRaid