rdf - Sparql - Applying limiting criteria to predicates

Question

I'm fairly new to RDF / Sparql, so apologies for any incorrect terminology, and also for the fairly terrible example that follows:

Given the following RDF dataset:

@prefix owl:   <http://www.w3.org/2002/07/owl#> .
@prefix rdf:   <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix e:     <http://www.example.com/#> .
@prefix foaf:  <http://xmlns.com/foaf/0.1/> .

e:Freemason a owl:Class .
e:Civilian a owl:Class .

e:Marty a e:Freemason .
e:Eugene a e:Freemason .
e:Mike a e:Freemason .
e:Alan a e:Freemason .

e:Paul a e:Civilian .

e:Marty foaf:knows e:Eugene .
e:Eugene foaf:knows e:Mike .
e:Eugene foaf:knows e:Paul .
e:Paul foaf:knows e:Alan .

I'm trying to identify friends-of-friends that e:Marty knows through other e:Freemasons only.

So:

• Marty knows Mike through Eugene, and they're all Freemason's so it's fine
• Marty knows Eugene, who has a Civilian friend Paul. Paul has a Freemason friend Alan. However, Marty doesn't have a "freemason only" path to Alan, so he should be excluded.

Here's the SPARQL query I have:

prefix e: <http://www.example.com/#>
prefix foaf:  <http://xmlns.com/foaf/0.1/>

SELECT *
{
  <http://www.example.com/#Marty> foaf:knows+ ?target .
  ?target a e:Freemason .
}

This returns:

 http://www.example.com/#Eugene
 http://www.example.com/#Mike
 http://www.example.com/#Alan

Here, Alan is included as he matches the is-a-freemason criteria.

How I do modify the query to exclude Alan?

Answer 1

I don't know the solution in pure SPARQL, sorry. In OpenLink Virtuoso's SPARQL-BI, the solution is this query

prefix e: <http://www.example.com/#>
prefix foaf:  <http://xmlns.com/foaf/0.1/>
select * 
where
  {
    { select ?orig ?target 
      where
       { ?orig   foaf:knows ?target . 
         ?target a          e:Freemason .
       } 
    } 
    option ( TRANSITIVE, 
             T_IN(?orig), 
             T_OUT(?target), 
             T_DISTINCT, 
             T_MIN(1)
           )
    filter ( ?orig = <http://www.example.com/#Marty> )
  }

-- with these results --

orig                               target
<http://www.example.com/#Marty>    <http://www.example.com/#Eugene>
<http://www.example.com/#Marty>    <http://www.example.com/#Mike>