Hieratic Category Query

[eluser]esra[/eluser]
[quote author="lightnb" date="1205644714"]Would it make sense then to have a root that isn't actually the root of the tree, but a logical device that exists for the sole purpose of tree integrity?

In other words, a root node, manually inserted, that never actually gets displayed or used in the application for any reason?[/quote]

With nested sets, you need a parent root node as the basis of each hierarchy. All MPTT or MIPTT queries are based on the existence of a root node. However, this does not mean that a single table could not store multiple hierarchies, each with their own root. You could create a second table called trees and add a foreign key to the hierarchies table to associate hierarchies of nodes with different trees (roots). Then it's possible to clone (copy) an existing hierarchy and append it to another hierarchy (tree) with a different foreign key. In this case, you could think of the hierarchies table itself as a root of all hierarchies (trees) contained within that table.

There are some other approaches to handling this based on the use of a level column, making it easier to copy and move branches of a tree about.

http://www.phpriot.com/articles/nested-trees-1