title: Retrieving Property of A Class of Entities In SPARQL (wikidata) Not As Expected
body: <p>Why does the following query:</p>
  
  <pre><code>SELECT ?item ?itemLabel ?o ?oLabel WHERE {
    ?item wdt:P31 wd:Q3918.
    ?item wdt:P131 ?o.
    SERVICE wikibase:label { bd:serviceParam wikibase:language "[AUTO_LANGUAGE],en". }
  }
  </code></pre>
  
  <p>not return <a href="https://www.wikidata.org/wiki/Q160302" rel="nofollow noreferrer">https://www.wikidata.org/wiki/Q160302</a>, when clearly this entity has a property P131: <a href="https://www.wikidata.org/wiki/Q160302#P131" rel="nofollow noreferrer">https://www.wikidata.org/wiki/Q160302#P131</a>?</p>
  
  <p>Including the OPTIONAL clause does:</p>
  
  <pre><code>SELECT ?item ?itemLabel ?o ?oLabel WHERE {
    ?item wdt:P31 wd:Q3918.
    OPTIONAL {?item wdt:P131 ?o.}
    SERVICE wikibase:label { bd:serviceParam wikibase:language "[AUTO_LANGUAGE],en". }
  }
  </code></pre>
  
  <p>But why?</p>