example_wd_federation.rq

# Folders from 20th Century Press Archives, extended with Wikipedia pages
#
PREFIX dct: <http://purl.org/dc/terms/>
PREFIX schema: <http://schema.org/>
PREFIX skos: <http://www.w3.org/2004/02/skos/core#>
PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX zbwext: <http://zbw.eu/namespaces/zbw-extensions/>
#
select *
where {
  # Persons of Irish nationality with publicly
  # accessible documents
  ?person skos:prefLabel ?personLabel ;
          dct:identifier ?personId ;
          schema:nationality "Ireland"@en ;
          zbwext:freeDocCount ?freeDocCount .
  filter(?freeDocCount > 0)
  #
  # Include information from the Wikidata Query Service
  service <https://query.wikidata.org/sparql> {
    # Wikidata items for the persons
    ?item wdt:P31 wd:Q5 ;
          wdt:P4293 ?personId ;
          wdt:P214 ?viafId .
    #
    # Add page links to the English Wikipedia
    ?page schema:about ?item ;
          schema:isPartOf <https://en.wikipedia.org/> .
  }
}
order by desc(?freeDocCount)

# Note: Queries with large datasets created before the service clause
# (e.g., asking for German persons instead of Irish) may take very long,
# because the intermediate results may be transmitted step by step to the
# external endpoint (implementation-dependent).
# It sometimes helps to reverse the inovocation order, i.e. to first
# query the external endpoint, and then filter the results with additional
# statements locally.