Gijs van Tulder

PhD student in machine learning   |   computer scientist   |   the Netherlands

Storing Hierarchical Data in a Database

Whether you want to build your own forum, publish the messages from a mailing list on your Website, or write your own cms: there will be a moment that you’ll want to store hierarchical data in a database. And, unless you’re using a XML-like database, tables aren’t hierarchical; they’re just a flat list. You’ll have to find a way to translate the hierarchy in a flat file.

Storing trees is a common problem, with multiple solutions. There are two major approaches: the adjacency list model, and the modified preorder tree traversal algorithm.

In this article, we’ll explore these two methods of saving hierarchical data. I’ll use the tree from a fictional online food store as an example. This food store organizes its food by category, by colour and by type. The tree looks like this:

This article contains a number of code examples that show how to save and retrieve data. Because I use that language myself, and many other people use or know that language too, I chose to write the examples in PHP. You can probably easily translate them to your own language of choice.

Continue reading on SitePoint.com

This is an archived copy of an article I wrote for SitePoint.com, a web development site, in April 2003.