I'm trying to query data from dbpedia by a country's name. I want it to find it whether there is a resource for that country or via its existence in wikiPageRedirects. Here is a working version:
PREFIX res: <http://dbpedia.org/resource/>
PREFIX ont: <http://dbpedia.org/ontology/>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
SELECT ?country ?capital ?label
WHERE {
{ res:Dominion_of_Canada ont:capital ?capital .
?capital rdfs:label ?label }
UNION
{ res:Dominion_of_Canada ont:wikiPageRedirects ?country .
?country ont:capital ?capital .
?capital rdfs:label ?label }
FILTER (lang(?label) = "en")
}
I'd like (if possible), to factor out the ?country. Is it possible to assign a resource to a variable such that the SPARQL query looks like the following?
SELECT ?country ?capital ?label
WHERE {
{ ?country EXISTS res:Dominion_of_Canada } # to get the idea across
UNION
{ res:Dominion_of_Canada ont:wikiPageRedirects ?country }
?country ont:capital ?capital .
?capital rdfs:label ?label .
FILTER (lang(?label) = "en")
}
As ever, speed is important, too. If the resource exists, then it'd be better if it skipped searching on wikiPageRedirects.
Checking whether a resource "exists" or not is a bit vague, since IRIs are just constant data. The question is really whether DBpedia contains any triples about a particular resource. In your case, you're wanting to know whether it it redirects to anything else, or if it has properties of its own. A property path of the form dbpedia:France dbpedia-owl:wikiPageRedirects* ?country is really probably the best way to do that. If there are no redirect links, then ?country is dbpedia:France, and if there are, then ?country is the value of the redirects. The only way to "check" is to look for those triples. I think that means you will end up with something like this (similar to what's shown in my answer to another question involving redirects):
select ?country ?anthem ?author {
#-- The only way to really "check" that the resource
#-- "exists" and is not a redirect, is by checking
#-- whether it has any redirect links. If it doesn't,
#-- then ?country is dbpedia-owl:France, like you want
#-- and if it does, then then you want to follow them.
dbpedia:France dbpedia-owl:wikiPageRedirects* ?country .
#-- I'm using anthem and author here because
#-- it doesn't look like there was reliable information
#-- about the capital.
?country dbpedia-owl:anthem ?anthem .
?anthem dbpprop:author ?author .
}