Question

If I have a SPARQL query say,

PREFIX foaf <http://xmlns.com/foaf/0.1/>
SELECT ?name
WHERE {
  ?x foaf:name ?name.
  ?x foaf:knows ?y.
}

to select the name of some x, who knows some y. How could I select only the names of those people who know exactly 3 other people (or any other number)?

Also, as a side question – is there a better title for this question? One which uses better terminology to clarify the problem?

Thanks

Answer 1

You could achieve that with SPARQL 1.1 and it new features: GROUP BY, HAVING and SUBQUERIES. Something like this would do the job:

SELECT ?name
WHERE {
    ?x foaf:name ?name .
    {
       SELECT ?x (count(?y) as ?count_y) WHERE {
              ?x foaf:knows ?y.
       } GROUP BY ?x 
         HAVING count(?y) > 3
    }
}

Unfortunately not all SPARQL engines support all these features together. That I know Jena/ARQ and Virtuoso support them.

If you are working with an SPARQL engine that doesn’t support these features then I recommend to run the query:

SELECT ?name
WHERE {
  ?x foaf:name ?name.
  ?x foaf:knows ?y.
}

… and programatically compute the rest of the logic that you need in the query with few lines of client-side code.