The tables might look like this: In this model, the employee data table represents the "parent" part of the hierarchy, while the computer table represents the "child" part of the hierarchy. In contrast to tree structures usually found in computer software algorithms, in this model the children point to the parents. As shown, each employee may possess several pieces of computer equipment, but each individual piece of computer equipment may have only one employee owner. Consider the following structure: In this, the "child" is the same type as the "parent". The hierarchy stating EmpNo 10 is boss of 20, and 30 and 40 each report to 20 is represented by the "ReportsTo" column. In Relational database terms, the ReportsTo column is a foreign key referencing the EmpNo column. If the "child" data type were different, it would be in a different table, but there would still be a foreign key referencing the EmpNo column of the employees table. This simple model is commonly known as the adjacency list model and was introduced by Dr. Edgar F. Codd after initial criticisms surfaced that the relational model could not model hierarchical data. However, the model is only a special case of a general adjacency list for a graph. See also Tree structure Hierarchical query Hierarchical clustering References External links Troels' links to Hierarchical data in RDBMSs Managing Hierarchical Data in MySQL (This page is from archive.org as the page has been removed from MySQL.com) Hierarchical data in MySQL: parents and children in one query Create Hierarchy Chart from Hierarchical Database Database models