Trees and hierarchical data in SQL

Here are some links to good articles on trees and hierarchical data in relational databases

Sitepoint article Storing Hierarchical Data in a Database By Gijs Van Tulder

Trees in SQL by Jan kneschke

Mysql.com article Managing Hierarchical Data in MySQL by Mike Hillyer (note: Article no longer available on mysql.com, but on archive.org)

Some Articles by Vadim Tropashko

Trees in SQL: Nested Sets and Materialized Path

Nested intervals tree encoding with continued fractions

he has more on his blog

and a book

Tropashko SQL Design Patterns: The Expert Guide to SQL Programming