SPARQL full aggregation on a group aggregation

Question

I have an Ontology where users can use one of five predicates to express how much they like an item.

The Ontology contains specific predicates that have a property calle

Answer 1

It's really helpful if you can provide data to work with in these examples that's minimal. That means data that doesn't have stuff we don't need in order to solve the problem, and that is pretty much as simple as possible. I think that How to create a Minimal, Complete, and Verifiable example might be useful for your Stack Overflow questions.

Anyhow, here's some simple data that should be enough for us to work with. There are two users who have made some ratings, and some similarities in the data. Note that I made the similarities directed; you'd probably want them to be bidirectional, but that's not really the main part of this problem.

@prefix : <urn:ex:>

:user1 :rated :a , :b .

:user2 :rated :b , :c , :d .

:a :similarTo [ :piece :c ; :value 0.1 ] ,
              [ :piece :d ; :value 0.2 ] .

:b :similarTo [ :piece :d ; :value 0.3 ] ,
              [ :piece :e ; :value 0.4 ] .

:c :similarTo [ :piece :e ; :value 0.5 ] ,
              [ :piece :f ; :value 0.6 ] .

:d :similarTo [ :piece :f ; :value 0.7 ] ,
              [ :piece :g ; :value 0.8 ] .

Now, the query just needs to retrieve a user and the pieces that they've rated, along with similar pieces and the actual similarity values. Now, if you group by the user and the similar piece, you end up with a groups that have a single similar piece, a single user, and a bunch of rated pieces and their similarity to the similar piece. Since all the similarity ratings are in a fixed range (0,1), you can just average them to get overall similarity. In this query, I've also added a group_concat to show which rated pieces the similarity value is based on.

prefix : <urn:ex:>

select
    ?user
    (group_concat(?piece) as ?ratedPieces)
    ?similarPiece
    (avg(?similarity_) as ?similarity)
where {
  #-- Find ?pieces that ?user has rated.
  ?user :rated ?piece .

  #-- Find other pieces (?similarPiece) that are
  #-- similar to ?piece, along with the
  #-- similarity value (?similarity_)
  ?piece :similarTo [ :piece ?similarPiece ; :value ?similarity_ ] .
}
group by ?user ?similarPiece

------------------------------------------------------------
| user   | ratedPieces         | similarPiece | similarity |
============================================================
| :user1 | "urn:ex:a"          | :c           | 0.1        | ; a-c[0.1]
| :user1 | "urn:ex:b urn:ex:a" | :d           | 0.25       | ; b-d[0.3], a-d[0.2]
| :user1 | "urn:ex:b"          | :e           | 0.4        | ; b-e[0.4]
| :user2 | "urn:ex:b"          | :d           | 0.3        | ; b-d[0.3]
| :user2 | "urn:ex:c urn:ex:b" | :e           | 0.45       | ; c-e[0.5], b-e[0.4]
| :user2 | "urn:ex:d urn:ex:c" | :f           | 0.65       | ; d-f[0.7], c-f[0.6]
| :user2 | "urn:ex:d"          | :g           | 0.8        | ; d-g[0.8]
------------------------------------------------------------