title: Postgres issue with Gount / Group By / date_trunc()
body: <p>I know that there are a couple of threads regarding this, but I read them all w/o any luck.</p>
  
  <p>I have the following query :</p>
  
  <pre><code>select coalesce(count(*)), date_trunc('month', generate_series(min(item.updated_at), max(item.updated_at), '1 month'))
              from item 
              group by date_trunc('month', item.updated_at) 
              order by date_trunc ;
  </code></pre>
  
  <p>but it only shows months where items where updated at. And it just skips months with 0 matches.</p>
  
  <p>I tried adding coalesce and generating the series with generate_series(), but it's still not working.</p>
  
  <p>Any clues?.</p>
  
  <p>Thanks a lot in advance.</p>