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sparqldbpedia

SPARQL: Select attribute of all resource subject of category

I have this SPARQL query to select all resources beloning to the Wikipedia category National_parks_in_California:

PREFIX dcterms: <http://purl.org/dc/terms/>

SELECT * WHERE {
  ?park dcterms:subject dbc:National_parks_in_California .
}

which returns 

park
http://dbpedia.org/resource/Death_Valley_National_Park
http://dbpedia.org/resource/Yosemite_National_Park
http://dbpedia.org/resource/Channel_Islands_National_Park
http://dbpedia.org/resource/Kings_Canyon_National_Park
http://dbpedia.org/resource/Lassen_Volcanic_National_Park
http://dbpedia.org/resource/Redwood_National_and_State_Parks
http://dbpedia.org/resource/Joshua_Tree_National_Park
http://dbpedia.org/resource/Pinnacles_National_Park
http://dbpedia.org/resource/Sequoia_National_Park

Is it possible to build a query to return along with the dbpedia resource page also the foaf:homepage of each resource?

I wrote 

PREFIX dcterms: <http://purl.org/dc/terms/>
PREFIX foaf: <http://xmlns.com/foaf/0.1/>

SELECT * WHERE {
  ?park dcterms:subject dbc:National_parks_in_California .
  ?webpage foaf:homepage ?park .
}

but it returns zero results.

Solution

  • If you open on of the results, you will see that each resource MIGHT have a foaf:homepage. Therefore, the direction of your query is wrong

    PREFIX dcterms: <http://purl.org/dc/terms/>
PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT * WHERE {
      ?park dcterms:subject dbc:National_parks_in_California .
      optional{
            ?park foaf:homepage ?homepage.
      }
}

    The reason I have used optional is that some resources do not have foaf:homepage.