How to implement a filtering feature with NSCoding as a database solution?

Question

I have an app which lets a user create and save categories (e.g. 'Feeling Happy/Sad') and a user can create, save Mood Notes by providing title, category, and the text/note.

I used NSCoding as a database solution, but I dont know all the details about NSCoding.

So I'm required to provide a feature where a user can tap on a category cell and see all the notes that have that category.

I dont know how to implement this feature using NSCoding, how to implement sorting/filtering, what to do if a user created categories that are identical, etc.

Also, going though some iOS tutorial, it said that

NSCoding doesn't support querying or creating complex object graphs

What does this line mean (not literally)? What can I do about implementing the requested feature?

Thank you!