Return a nested data structure from a SPARQL query

Question

If I have a graph with this kind of structure:

@prefix  :        .
@prefix  rdf:     .
         


        

Answer 1

You're conflating the query result itself (essentially an abstract table structure) with the syntax in which that result is written (in your case, a customized nested JSON structure).

Don't try to do tricks with group concatenation in this case. Just do this query:

SELECT ?given ?family ?friend_given ?friend_family
WHERE {
  ?person foaf:givenName ?given ;
          foaf:familyName ?family .
  ?person :knows ?friend .
  ?friend foaf:givenName ?friend_given ;
          foaf:familyName ?friend_family .
}
GROUP BY ?family ?given

Which results in a result like this:

given  family  friend_given friend_family
-------------------------------------------- 
Alice  Lidell  Bob          Doe
Alice  Lidell  Hwa          Choi

And then let a custom streaming result writer write the result to the nested syntax format you require. Given that the query groups by name, the writer can safely assume that subsequent rows with the same given and family names "belong together".

Alternatively, use a CONSTRUCT query instead of a SELECT, and post-process the retrieved RDF graph (which accurately represents the tree structure you're after).