How to filter a variable by a property included in the variable in SPARQL?

Question

I want to write a SPARQL query that would return the first name of a person based on the ranking of the name on Wikidata.

For example, let's say I want the second first name of Mozart (Chrysostom).

This is what I have so far (Mozart Wikidata ID is Q254, first name's property is P735, with P1545 giving the ordinal position of the name):

  SELECT DISTINCT ?full_name ?full_nameLabel ?first_nameLabel ?rank 
    WHERE
                      {
                        VALUES ?full_name {wd:Q254} .
                        ?full_name p:P735 [pq:P1545 ?rank] ;
                        p:P735 [ps:P735 ?first_name] ; 
                        FILTER regex(?rank, "2")
                        SERVICE wikibase:label { bd:serviceParam wikibase:language "en" }
                      }

However here the filter only applies to the rank variable, and not on the first_name variable:

Query result:

I think that the problem comes from the fact that the rank property is a sub-element of the first_name property. Would you know of a way to filter the first_name variable by the rank variable?

Answer 1

SELECT DISTINCT ?id ?idLabel ?first_nameLabel ?rank
    WHERE {
        VALUES ?id {wd:Q254} .
        ?id p:P735 [
            pq:P1545 ?rank; 
            ps:P735 ?first_name
        ] 
        FILTER(?rank = "2")
        SERVICE wikibase:label { bd:serviceParam wikibase:language "en" }           
    }