Remove the { } around the 3 first lines and it should work fine.
Le dim. 25 nov. 2018 à 15:52, Markus Kroetzsch <
[email protected]> a écrit :
> Hi,
>
> I am puzzled by the behaviour of a SPARQL query. Maybe there is an error
> with BlazeGraph here, but hopefully I am just overlooking something.
>
> The query is as follows: http://tinyurl.com/y95jpmhq
>
> SELECT ?item ?birthdate ?spouse
> WHERE
> {
> { ?item wdt:P569 ?birthdate
> FILTER (year(?birthdate)>1900)
> ?item wdt:P26 []
> } OPTIONAL {
> ?item wdt:P26 ?spouse
> FILTER (year(?birthdate) = 1947)
> }
> # FILTER (year(?birthdate) = 1947) ## For testing: works correctly
> } LIMIT 1000
>
> What this should do: "Select married people born after 1900, and,
> optionally, also select their spouses, but only for people born in
> 1947." What BlazeGraph does is: "Select married people born after 1900;
> never select any spouses, even if the person is born in 1947".
>
> The 1000 results should contain lines for 1947 births, so you can see
> they have no spouse. The commented out filter at the bottom can be used
> instead of the inner filter to verify that the condition has no typos
> and really matches some of the items.
>
> It seems that BlazeGraph gets the scope of ?birthdate wrong here, and
> rather processes the whole query inside out, applying the FILTER to the
> optional pattern (where ?birthdate is not bound) and then using the
> (empty) result in a binary LeftJoin operation. In reality, LeftJoin in
> the SPARQL algebra is a ternary operator that applies the FILTER to the
> Join of both sides to determine if we have an optional match or not:
>
> * See "Definition: LeftJoin" in Section 18.5 of the spec [1].
>
> Filters within optional patterns become the third parameter in the
> LeftJoin operation when translating queries as in my example:
>
> * See example "{ ?s :p1 ?v1 OPTIONAL {?s :p2 ?v2 FILTER(?v1<3) } }" in
> Section 18.2.3 of the spec [1].
>
> Is my interpretation correct or did I overlook something? Is this a
> known problem?
>
> Cheers,
>
> Markus
>
> [1] https://www.w3.org/TR/sparql11-query
>
> --
> Prof. Dr. Markus Kroetzsch
> Knowledge-Based Systems Group
> Center for Advancing Electronics Dresden (cfaed)
> Faculty of Computer Science
> TU Dresden
> +49 351 463 38486
> https://kbs.inf.tu-dresden.de/
>
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