title: Return list of elements in group by query on Apache druid
body: <p>Having information stored as:</p>
  
  <pre><code>id;name;item;
  1;one;1;
  2;two;2;
  1;one;3;
  </code></pre>
  
  <p>I want to group the items by id so I obtain:</p>
  
  <pre><code>id;items
  1,[1,3]
  2,[2]
  </code></pre>
  
  <p>Is there a way to achieve this on Apache Druid? I can only find examples of group by queries with sum, count, etc. </p>
  
  <p>For example, in Oracle I would use JSON_ARRAYAGG or LISTAGG functions.</p>