To gain some experience, I am trying to make an expert system that can answer queries about the animal kingdom. However, I have run into a problem modeling the domain. I originally considered the animal kingdom hierarchy to be drawn like -animal -bird -carnivore -hawk -herbivore -bluejay -mammals -carnivores -herbivores This I figured would allow me to make queries easily like "give me all birds", but would be much more expensive to say "give me all carnivores", so I rewrote the hierarchy to look like: -animal -carnivore -birds -hawk -mammals -xyz -herbivores -birds -bluejay -mammals But now it will be much slower to query "give me all birds." This is of course a simple example, but it got me thinking that I don't really know how to model complex relationships that are not so strictly hierarchical in nature in the context of writing an expert system to answer queries as stated above. A directed, cyclic graph seems like it could mathematically solve the problem, but storing this in a relational database and maintaining it (updates) would seem like a nightmare to me. I would like to know how people typically model such things. Explanations or pointers to resources to read further would be acceptable and appreciated.