Always one root category

The category tree should always have exactly one root element. Not more, not less. 
The name of the shop could be the root element. Make sure that the root element gets an ID.

Attributes

If a product has two values for a single attribute, for example blue and red, then these two need to be taken as two separate attributes in the list. 

Example → 

Product in multiple categories

A product can be connected to multiple categories but must be connected to at least 1. Make sure these categories also exist in the category structure.

Example →