The hierarchical data structure is one of the most common data structures in programming. It represents a set of data where each item (except the root) has one parent node and one or more children nodes. Web programmers use hierarchical data for a number of applications, including content management systems, forum threads, mailing lists, and e-commerce product categorization. In this tutorial , I will explain how to manage hierarchical data using MySQL and PHP. Specifically, I will explore the two most popular hierarchical data models: adjacency list model and nested set model.

They start with the adjacency list model, showing how to do a left join on the table on the parent ID to record ID and then filter through them, merging them into a main array as needed. The nested set model is slightly different and involves two new columns, a "lft" and "rgt", that point you to the records around the current one. This method makes the actual SQL query to find the structure a bit simpler, but isn't quite as flexible as the first method.