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<Nested set in MySQL ⁄ >

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There seems to be massive confusion about the concept behind nested set and when/how it should be used. In this article I will attempt to remove some of the mystery behind nested set, explain some of the magic and provide a few easy to understand examples for implementing and working with nested set in MySQL.

Before I get started I should note that there is a reasonably extensive article at MySQL on this topic. What I am trying to achieve here is an easier to understand explanation to the same problem.

What is nested set? When should I use it?

The nested set model is a way of efficiently storing and retrieving hierarchical data in a database. The model requires "sequence" data to be stored with each record to indicate it's order and depth within a hierarchy. That stored sequence data can then be used to retrieve the data or parts of the data along with the associated depth information without the need to use multiple queries or sub-queries.

Nested set can be used whenever you need to store and retrieve any kind of hierarchical data. Regardless of whether it is single parented or multi-parented data. For example, if you have a "family tree" that you want to store in a database or if you need to store an organisational structure.

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