In his previous book, (SQL for Smarties, 2e, MK, 1999) Joe Celko wrote two chapters on programming techniques for representing trees and hierarchies in SQL. Ever since then, he has been bombarded with questions requesting more explanation of these topics. Joe has decided to meet the demand for this information by addressing the topic in depth in his new full-length book. Trees and hierarchies are a way to organize information and appear everywhere in computer science, from indexing structures (i.e. B-Tree indexing) to encoding schemes (i.e. Dewey Decimal system for libraries) to hierarchical databases. Even XML and related markup languages-which interact with databases-are based on tree structures. Every SQL programmer is faced with the challenge of creating these structures, which are not easy to master and have far-reaching programmatic effects.