sparql query returning nothing - sparql

Question

Why this SPARQL not returning any value
PREFIX ontology: <http://dbpedia.org/ontology/>
SELECT ?Abstract
WHERE
{
<http://dbpedia.org/resource/Cologne> <http://dbpedia.org/ontology/wikiPageRedirects> ?page .
?page <http://dbpedia.org/ontology/abstract> ?Abstract.
FILTER (lang(?Abstract)='en')
}

Answer 1

Two reasons. Firstly, there are no redirects of Cologne, but there are redirects to it. So:
PREFIX ontology: <http://dbpedia.org/ontology/>
SELECT *
WHERE
{
<http://dbpedia.org/resource/Cologne>
<http://dbpedia.org/ontology/wikiPageRedirects> ?page .
}
returns nothing, whereas:
PREFIX ontology: <http://dbpedia.org/ontology/>
SELECT *
WHERE
{
?page
<http://dbpedia.org/ontology/wikiPageRedirects>
<http://dbpedia.org/resource/Cologne> .
}
does work.
Secondly none of these redirects appears to have an abstract. Cologne itself does, so you could just use that:
PREFIX ontology: <http://dbpedia.org/ontology/>
SELECT ?Abstract
WHERE
{
<http://dbpedia.org/resource/Cologne>
<http://dbpedia.org/ontology/abstract> ?Abstract.
FILTER (lang(?Abstract)='en')
}

Answer 2

It does not return any value because nothing matches your query. If you dereference the URI for Cologne, you can see that there is no wikiPageRedirects property. dbpedia:Cologne does not redirects to anything, but many resources redirect to dbpedia:Cologne. However, the resources that redirect to Cologne do not have an abstract. In fact, this is quite normal: in Wikipedia, you cannot edit a page that redirects to another one, as you are necessarily redirected to the other. So when dbpedia extracts data from Wikipedia, of course there is nothing about the redirected pages.