Effective Tree Structures In Relational Databases

Joe Celko explains the Nested Set Model. It is an alternative way of storing tree-structured data in relational databases. The main benefit is that it allows you to make queries that would be hard or impossible if you follow the classic approach i.e. storing the parent id in each node. One example: with the nested set model it is easy to select all children recursive of a given node.

Fog Creek Software uses this approach in FogBugz.