Hi,
There is a general caveat when building SPARQL Unions of Subqueries in
Virtuoso which I believe is unnecessary.
The following query works fine.
select distinct ?s ?text where {
{ ?s rdfs:label ?text } union { ?s rdfs:comment ?text }
} limit 10
But assume I want to complicate it a bit by using subqueries ( in this
case it makes no sense, but there are definitely use cases for this,
like combing results of complex queries )
select distinct ?s ?text where {
{ select ?text where {?s rdfs:label ?text} } union { select ?text
where {?s rdfs:comment ?text} }
} limit 10
The latter query will yield an error:
37000 Error SP031: SPARQL compiler: Variable 's' is used in the query
result set but not assigned
So, it follows that I need to bind the variables at least once in the
outer pattern.
Fine, I can do that:
select distinct ?s ?text where {
?s ?p ?text .
{ select ?text where {?s rdfs:label ?text} } union { select ?text
where {?s rdfs:comment ?text} }
} limit 10
And it works ;)
But, here's the observation: Why do I HAVE to declare the variables outside?
Isn't there any way that the compiler can detect which variables are
being referenced in the select projections part?
When dealing with complex queries I usually find myself writing dummy
BGPs, for example:
?interestingSubject ?xx1 ?xx2 .
?xx3 ?xx4 ?interestingObject .
{ subquery1... } union { subquery2...} union {subquery3...}
Will enabling this "automatically" introduce ambiguity?
Regards,
A
--
Aldo Bucchi
skype:aldo.bucchi
http://www.univrz.com/
http://aldobucchi.com/
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