SPARQL select does not work as expected - sparql

Question

If I run this query
SELECT ?a ?b ?c ?g ?maxtrust
WHERE
{
{
SELECT ?a (MAX(?t) AS ?maxtrust)
WHERE{
GRAPH ?g { ?a a gr:productorservicemodel }
GRAPH <uri:trust> { ?g exepo:trust ?t}
}
GROUP BY ?a
}
GRAPH ?g {?a ?b ?c}
GRAPH <uri:trust> { ?g exepo:trust ?maxtrust}
}
i get this response:
| a | b | c | g | maxtrust |
|-----------|------------------|--------------------------|-----------|----------|
| uri:prodA | rdf:type | gr:ProductOrServiceModel | uri:alice | 1.0 |
| uri:prodA | exe:EarCoupling | Intraaural | uri:alice | 1.0 |
| uri:prodA | exe:WearingStyle | In-ear | uri:alice | 1.0 |
| uri:prodB | rdf:type | gr:ProductOrServiceModel | uri:bob | 0.5 |
| uri:prodB | exe:EarCoupling | Extraauricolare | uri:bob | 0.5 |
since I'm only interested in the relation between a and g I guessed that this query will have done the trick:
SELECT ?a ?g
WHERE
{
{
SELECT ?a (MAX(?t) AS ?maxtrust)
WHERE{
GRAPH ?g { ?a a gr:productorservicemodel }
GRAPH <uri:trust> { ?g exepo:trust ?t}
}
GROUP BY ?a
}
GRAPH ?g {?a ?b ?c}
GRAPH <uri:trust> { ?g exepo:trust ?maxtrust}
}
I would expected this result:
| a | g |
|-----------|-----------|
| uri:prodA | uri:alice |
| uri:prodA | uri:alice |
| uri:prodA | uri:alice |
| uri:prodB | uri:bob |
| uri:prodB | uri:bob |
instead I got this:
| a | g |
|-----------|-----------|
| uri:prodA | uri:alice |
| uri:prodA | uri:alice |
| uri:prodA | uri:alice |
What is going on? is my understanding of the SPARQL logic completely wrong?
edit: more info
the datasets are:
alice (GRAPH uri:alice):
uri:prodA
a gr:ProductOrServiceModel;
exe:EarCoupling "Intraaural"^^xsd:string ;
exe:WearingStyle "In-ear"^^xsd:string .
bob (GRAPH uri:bob):
uri:prodA
a gr:ProductOrServiceModel;
exe:EarCoupling "Intraauricolare"^^xsd:string .
uri:prodB
exe:WearingStyle "extraauricolare"^^xsd:string .
trust (GRAPH uri:trust):
uri:alice exe:trust "1.0"^^xsd:float .
uri:bob exe:trust "0.5"^^xsd:float .
I'm using stardog as triplestore

Answer 1

if you're only interested in the relation between ?a and ?g your query could be much simpler:
select distinct ?a ?g where {
graph ?g { ?a [] [] }
}