Category in Sales Order Grid

[jjbigfish]

I'm looking to add "Category" to the admin sales grid, I would like to list the categories of the products purchased in the sales > orders grid view in the admin.

I can create the column in the grid.php file, but I don't know how to create the collection based on the products purchased.

Any help would be appreciated.

Thanks

[dahol]

I have sort of the same problem, as I don't know how to add "product-number" from "catalogue -> products" to the "sales -> order" grid