How do I represent a Tree Structure in a mySQL table - mysql

Question

The tree has the following characteristics:
Each node can have multiple parents and multiple children.
The Parent nodes of a Node can have different depth.
Example
I am trying to represent a category structure such as the following:
Desktop and Mobile Applications
Desktop and Mobile Applications->Android Apps
Desktop and Mobile Applications->Android Apps->Games
Desktop and Mobile Applications->Android Apps->Games->Action
Desktop and Mobile Applications->Games
Desktop and Mobile Applications->Games->Action
Desktop and Mobile Applications->Games->Adventure
Desktop Applications
Desktop Applications->Games
Desktop Applications->Games->Action
Desktop Applications->Games->Adventure
IPhone Applications
Desktop Applications->Games
Desktop Applications->Games->Action
Desktop Applications->Games->Adventure
Tried using the Nested Set Algorithm and I end up with multiple "Games" categories with different categoryIDs and at different depths.
Any help with this will be much appreciated.

Answer 1

The simple way is to structure a table like:
Categories
CategoryID
ParentID
Name
Your data would look like:
1, 0, 'Desktop and Mobile Apps'
2, 1, 'Android Apps'
3, 2, 'Games'
4, 3, 'Action'
5, 1, 'Games'
6, 5, 'Action'
7, 5, 'Adventure'
8, 0, 'Desktop Apps'
9, 8, 'Games'
You would query it like:
select * from Categories where ParentId = 1 which would return Android Apps and Games. To get the sub categories of games you would do select * from Categories where ParentId = 5 which would return action and adventure.
update
In order to associate a single item with multiple categories you will want one additional table:
xref_CategoriesItems
CategoryId
ItemId
This would allow any single item to be associated with multiple categories. Let's say you have a desktop app that needs to appear with both Desktop Apps > Games and Desktop and Mobile Apps > Games.
Your table would have the following data for item 1:
3, 1
9, 1
When seeing what items were in a specific category you would do the following:
select I.*
from items I
inner join xref_CategoriesItems XCI on (XCI.ItemId = I.ItemID)
WHERE (XCI.Category = #CategoryId)
To see which categories a specific item falls under:
select C.*
from categories C
inner join xref_CategoriesItems XCI on (XCI.CategoryId = C.CategoryId)
where (XCI.ItemId = #ItemId)
The query for all items under a specific category is a little more complex if you need all of the child records. Basically you need to do a recursive join the xref_categories with the categories to get the children. I don't remember how to express that in MySQL's version of sql; however the following might be good to know: Using MySQL query to traverse rows to make a recursive tree

Answer 2

What you are asking for is not really a tree, but a graph. Especifically it's a Directed Acyclic Graph (DAG). Here it's a link speaking about storing DAGs in a relational database.
It might be easier to just go with a traditional tree structure for the categories, and allow for items to be in multiple categories with the help of a item/category linking table.

Answer 3

An alternative approach would be to use tags on whatever items you are storing instead of categories. From the example you give, that may even be more appropriate - instead of "Desktop and Mobile Applications", "Desktop Applications" and "Mobile Applications", you would use the "Desktop" and "Mobile" tags. An item with both would then naturally fall into the first category.
Ref: How to store tags in MySQL tags, one field in total or one filed for each tag?