PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
SELECT ?name ?email
WHERE
{ 
   ?doc rdf:type foaf:Document ; 
		 foaf:maker ?author .
   ?author foaf:knows ?someone .
   ?someone foaf:name ?name . 
   OPTIONAL { ?someone foaf:mbox ?email . } 
 }
