Gijs van Tulder

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.

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