SPARQL Querying Data Linked Between Two Separate Graphs - sparql

Question

I have a graph of Persons stored in a StarDog instance, and a graph of Addresses in a Fuseki instance. One of the Person instances has a hasAddress relationship with an Address.
I wish to make a SPARQL query that simply returns all the Persons who have an address, as well as the address at which they live. This query is going to sent to the StarDog instance (the endpoint with the Person graph). The query I am using is
prefix testOnt: <http://www.example.org/test-ontology#>
SELECT ?person ?address
WHERE {
?person testOnt:hasAddress ?address
}
However, this returns no results.
I am initialising the StarDog database using this bit of TTL:
:SomeDude rdf:type owl:NamedIndividual , :Person;
:hasAddress :Address1.
Where Address1 is defined in the TDB on the Fuseki side.
I just don't know how I'm meant to reference the second graph when I make a query to the other.
Thanks for any help, and I can clarify any points in the comments.

Answer 1

Your query to retrieve just ?person testOnt:hasAddress ?address should work against Stardog but would not return address information stored in Fuseki. If this query is not working then make sure the namespaces you use in the data and the query match.
In order to get people instances from Stardog and their addresses from Fuseki you will need a federated SPARQL query that uses the SERVICE keyword. Assuming namespaces are correct, your query should look like this:
prefix testOnt: <http://www.example.org/test-ontology#>
SELECT ?person ?address
WHERE {
?person testOnt:hasAddress ?address
SERVICE <http://fuseki/location> {
?address ?p ?o # use a more specific BGP here if necessary
}
}