Hierarchies - Topological Ordering Topics (Nested Sets) - Examples
These example scripts describe the technique of topological ordering of hierarchies. They start by setting up a working set of tables and continue with a working example of the enumeration algorithm followed by use cases of ancestor-descendent queries and computational hierarchies.
This material is referenced by presentation E12 of the IDUG EMEA 2010.
Take a look at the The Relational Taboo page for the original article circa 1992. Also refer to Joe Celko's books for the basics of the technique.