I posted to this list some time ago about working with a hierarchical category structure. I had great difficulty with my problem and gave up for a time. I recently returned to it and resolved a big part of it. I have one step left to go, but at least I have solved this part.
Specifically, I wanted to sum and group expenses according to categories, not just at the bottom tier, but at higher tiers, so as to show more summarized information. A CEO primarily wants to know the sum total for all the business units, yet have the ability to drill down to more detailed levels if something is unusually high or low. In my case, I could see the details, but not the summary. Well now I can summarize by what I refer to as the 2nd-level categories.
Anyway, I hope this helps someone, as I have come to appreciate - and I mean really appreciate - the challenge of working with hierarchical structures in a 2-dimensional RDBMS. If anyone sees something I should explain better or in more depth, please let me know.