When I started looking into managing hierarchical data I found a lot of short articles that provided bits and pieces of the big picture, but nothing gave a good in-depth review of everything I needed to know to handle hierarchies.

One trend that began to develop was that the best information on the subject was coming from Joe Celko, a rather prolific author/speaker on all things SQL.

Joe Celko has a book in publication called SQL For Smarties, and this book had some basic information on handling hierarchies, which he followed up with Trees and Hierarchies in SQL for Smarties, a truly excellent book on the subject.

This book is not for the faint of heart, but it really is a must-read for those who are looking to handle hierarchies in SQL.

The book only addresses MySQL once, and while in that section it repeats Joe's opinion of MySQL not being a real database, users of MySQL 5 can rest assured that the stored procedure examples in the book work without modification (thanks to Joe and MySQL 5 both using the standard PL/PSM stored procedure syntax).

Overall this is an excellent book for those looking to implement hierarchical data in SQL.