Most users at one time or another have dealt with hierarchical data in a SQL database and no doubt learned that the management of hierarchical data is not what a relational database is intended for. The tables of a relational database are not hierarchical (like XML), but are simply a flat list. Hierarchical data has a parent-child relationship that is not naturally represented in a relational database table.

For our purposes, hierarchical data is a collection of data where each item has a single parent and zero or more children (with the exception of the root item, which has no parent). Hierarchical data can be found in a variety of database applications, including forum and mailing list threads, business organization charts, content management categories, and product categories.

In a hierarchical data model, data is organized into a tree-like structure. The tree structure allows repeating information using parent/child relationships. For an explanation of the tree data structure, see here http://en.wikipedia.org/wiki/Tree_data_structure

There are three major approaches to managing tree structures in relational databases, these are:

• the adjacency list model
• the nested set model (otherwise known as the modified pre-order tree traversal algorithm)
• materialized path model

These are explained in more detail in the following chapters, or see
http://www.dbazine.com/oracle/or-articles/tropashko4, http://dev.mysql.com/tech-resources/articles/hierarchical-data.html