Adjacency List Design AKA Recursion

Hierarchical queries in MySQL: http://explainextended.com/2009/03/17/hierarchical-queries-in-mysql/

http://en.wikipedia.org/wiki/Adjacency_list

http://www.sqlsummit.com/AdjacencyList.htm

http://explainextended.com/2009/07/22/hierarchial-queries-in-mysql-identifying-trees/

http://www.codeproject.com/Articles/162368/Implementing-a-Tree-Structure-with-Database

http://onlamp.com/pub/a/onlamp/2004/08/05/hierarchical_sql.html?page=2

https://github.com/take-five/acts_as_ordered_tree