Hi Ivan,
On Thu, Oct 15, 2009 at 6:08 AM, Ivan Mikhailov
<[email protected]> wrote:
> Hello Aldo,
>
>> Isn't there any way that the compiler can detect which variables are
> being referenced in the select projections part?
>
> It detects, but you write "select ?text", not "select ?s ?text" in subqueries.
But try it:
select distinct ?text where {
{ select ?text where {?s rdfs:label ?text} } union { select ?text
where {?s rdfs:comment ?text} }
} limit 10
You'll get an error as well:
http://bit.ly/3vNARm
>
> Like in SQL, Datalog and other similar languages, variables inside
> subexpressions are not in the surrounding scope, you should place ?s to
> the inner SELECT result-list to refer to it at the upper level.
> Similarly, ?text is usable not because it appears in { ?s
> rdfs:label ?text } but because it is found after SELECT. It could be,
> say,
> { select (?inner_s) as ?s (?inner_text) as ?text
> where {?inner_s rdfs:label ?inner_text} }
> to provide values for ?s and ?text even if there are no such variables
> in subquery at all.
Yes, this part is clear.
The way I see it is that all variables projected ( select'ed ) by the
inner expression become "potentially" available for the outer
expression.
This only happens IF the outer expression binds them as well in the
conditions clause. ( it doesn't suffice for it to "select" them ).
It is that IF that makes me mad every now and then.
>
> Next version of SPARQL compiler may contain special check for
> "namesakes" in different scopes, to keep the author warned.
The author says: "All warnings are welcome!" ;)
Regards,
A
>
> Best Regards,
> Ivan.
>
> On Thu, 2009-10-15 at 05:33 -0300, Aldo Bucchi wrote:
>> 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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