This series of articles is about implementing a nested tree using PHP and PostgreSQL. The theory and code in these articles also applies to MySQL, not just PostgreSQL.
An N-Level nested tree is a way to represent hierarchical data, such as a list of directories and files in your operating system, or the listings in a directory such as the Open Directory Project. There can be any number of levels and any number of elements within each level.
The goal is to be able lookup data in the tree as quickly as possible with minimal strain on resources. We will implementing this using the nested tree model by Joe Celko.