Storing Hierarchical Data in a Database
http://www.sitepoint.com/article/hierarchical-data-database

Van Tulder, Gijs
SitePoint
2004

Abstract:

Whether you want to build your own forum, publish the messages from a mailing list on your Website, or write your own CMS: there will be a moment that you'll want to store hierarchical data in a database. And, unless you're using a XML-like database, tables aren't hierarchical; they're just a flat list. You'll have to find a way to translate the hierarchy in a flat file.