title: Group concat not working
body: <p>I've written a query without using group_concat and get 9 rows returned, one row for each occupation that Einstein held.</p>
  
  <p>When I add group_concat on the occupation column, that column is null. I don't understand what I'm doing wrong here. What I expect to see is 1 row with all 9 occupations in the occupations column.</p>
  
  <p>Here is the simple query.</p>
  
  <pre><code>SELECT ?item ?itemLabel ?genderLabel (GROUP_CONCAT(?occupationLabel) AS ?occupations)
  WHERE {
    ?item wdt:P31 wd:Q5.
    ?item ?label "Albert Einstein"@en.
    ?item wdt:P21 ?gender .
    ?item wdt:P106 ?occupation .
    SERVICE wikibase:label { bd:serviceParam wikibase:language "en". }
  }
  GROUP BY ?item ?itemLabel ?genderLabel
  </code></pre>
  
  <hr>
  
  <p><strong>EDIT:</strong></p>
  
  <p>Here is the code that is producing the duplicate values.</p>
  
  <pre><code>SELECT ?item ?itemLabel ?genderLabel (GROUP_CONCAT(?occupationLabel) AS ?occupations)
  WHERE {
    ?item wdt:P31 wd:Q5.
    ?item ?label "Albert Einstein"@en.
    ?item wdt:P21 ?gender .
    OPTIONAL {
      ?item wdt:P106 ?occupation .
      ?occupation rdfs:label ?occupationLabel
      FILTER(LANGMATCHES(LANG(?occupationLabel), 'en'))
    }
    SERVICE wikibase:label { bd:serviceParam wikibase:language "en". }
  }
  GROUP BY ?item ?itemLabel ?genderLabel
  </code></pre>
  
  <p>Running this query gives me the following:</p>
  
  <blockquote>
    <p>professor professor physicist physicist inventor educationalist
    educationist university teacher academic science writer non-fiction
    writer philosopher of science theoretical physicist</p>
  </blockquote>
  
  <p>professor and physicist are duplicated</p>
  
  <hr>
  
  <p><strong>2nd EDIT</strong></p>
  
  <p>Also worth noting is that when I modify the query to not use <code>rdfs:label</code>, I get the right concated result in the occupation column (I've added the parentheses and the labels to the URL):</p>
  
  <pre><code>http://www.wikidata.org/entity/Q121594 (professor) 
  http://www.wikidata.org/entity/Q169470 (physicist) 
  http://www.wikidata.org/entity/Q205375 (inventor) 
  http://www.wikidata.org/entity/Q1231865 (educationalist)
  http://www.wikidata.org/entity/Q1622272 (university teacher)
  http://www.wikidata.org/entity/Q3745071 (science writer)
  http://www.wikidata.org/entity/Q15980158 (non-fiction writer)
  http://www.wikidata.org/entity/Q16389557 (philosopher of science)
  http://www.wikidata.org/entity/Q19350898(theoretical physicist)
  </code></pre>
  
  <p>So, I guess my question now is, is it possible to get one label per ID?</p>