On Oct 19, 2015, at 11:26 AM, Miguel <miguel.ceri...@gmail.com> wrote:
>
> Hi Gary,
> SERVICE blocks and sub-queries have different semantics relating to the
> variable scope.
>
> The variables in a sub-query are in a scope disjoint from the enclosing one,
> unless they are projected in the SELECT clause.
> So in the first example the variable ?time1 in the sub-query is distinct from
> the variable ?time1 in the main query.
> In this case you are correct, the inner ?time1 won't never be bound.
>
> The variables in a SERVICE blocks instead, share the same scope as the
> enclosing query.
> So in the second example there is only a ?time1 variable, referenced both
> inside and outside the SERVICE block.
> In this case ?time1 main perfectly be bound.
> Of course, if it is actually bound or not depends on the data.
I don’t think this is right. The bottom-up semantics of SERVICE (and SPARQL in
general) means that the service block will be evaluated independently, and then
joined with any results from the basic graph pattern that precedes it.
>> select distinct * {
>> ?person a ex:Person .
>> ?person ex:startTime ?time1 .
>>
>> service <http://example.com/foo> {
>> ?otherPerson a ex:Person .
>> ?otherPerson ex:startTime ?time2 .
>> FILTER (?time2 > ?time1)
>> }
>> }
>
Based on the description of “ Evaluation of a Service Pattern”[1]:
"""
...the multiset of solution mappings corresponding to the results of executing
query SELECT * WHERE Q against the service endpoint with IRI iri where Q is the
serialization of P in SPARQL syntax, in case of a successful service invocation
according to the SPARQL protocol...
"""
the evaluation here is going to send the SPARQL query:
SELECT * WHERE {
?otherPerson a ex:Person .
?otherPerson ex:startTime ?time2 .
FILTER (?time2 > ?time1)
}
to the endpoint at <http://example.com/foo>. And since this query will always
have ?time1 unbound at the point of evaluation, no results will be returned.
.greg
[1]
http://www.w3.org/TR/2013/REC-sparql11-federated-query-20130321/#algebra_service
