database - Hierarchical Data - Nested Set Model: MySql

Question

I am just learning how to implement the Nested Set Model but still have confusion with a certain aspect of it involving items that may be part of multiple categories. Given the example below that was pulled from HERE and mirrors many other examples I have come across...

How do you avoid duplication in the DB when you add Apples since they are multi-colored (i.e. Red, Yellow, Green)?

Answer 1

You do not avoid duplications and the apple (or a reference to the apple) will be placed twice in your tree otherwise it won't be a tree but rather a graph. Your question is equally applicable if you build a... Swing JTree or an HTML tree ;).

The nested set model is just an efficient way to push and traverse a tree structure in a relational DB.It is not a data structure itself. It's more popular among MySQL users since MySQL lacks functionality for processing tree structures (e.g. like the one that Oracle provides).

Cheers!