The nested sets solution is one of the most beautiful solutions I’ve seen for modeling trees.
Joe Celko has extensive coverage of the Nested Sets model in his writings. You can find Joe Celko’s coverage of nested sets in his book Joe Celko’s Trees and Hierarchies in SQL for Smarties (Morgan-Kaufmann, 2004).
The main advantages of the nested sets solution are simple and fast queries, which I’ll describe later, and no level limit. Unfortunately, however, with large data sets the solution’s practicality is usually limited to static trees. For dynamic environments, the solution is limited to small trees (or forests of small trees). ...