On 09.09.2015 00:07, Markus Krötzsch wrote:
On 08.09.2015 23:56, Denny Vrandečić wrote:
Anyone an idea why this query has a trouble when I add the OPTIONAL
keyword?
*http://tinyurl.com/pgsujp2*
Doesn't look much harder than the queries in the examples.
Looking at the bottom of the exceptions, you can see that the system
simply refuses to run the query. The problem seems to be with the
labelling service. If you don't select the label for ?head, it works fine:
http://tinyurl.com/p6rfpgv
It seems the implementation of the label service needs some improvement
to support unbound variables (which should then return unbound labels,
rather than throw a runtime exception ;-).
P.S. I am not convinced yet of this non-standard extension of SPARQL to
fetch labels. Its behaviour based on the variables given in SELECT seems
to contradict SPARQL (where SELECT is applied to the results of a query
*after* they are computed, without having any direct influence on the
meaning of the WHERE part).
A simple UI that would query the Web API on demand might be better, and
not put any load on the query service. Considering that the main query
service is not the W3C conforming SPARQL endpoint (which gives you raw
results), one could as well have some JavaScript there to beautify the
resulting item URIs based on client-side requests. Maybe some consumers
really need to get labels from SPARQL, but at least the users who want
to see results right away would not need this.
Markus
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