title: Union sentence gives error only referencing one column
body: <p>I'm using a table called Task and a sql union query for retrieving some tasks. All the info can be found here:</p>
  
  <p><a href="http://sqlfiddle.com/#!2/786e1/2" rel="nofollow">http://sqlfiddle.com/#!2/786e1/2</a></p>
  
  <p>My problem is this:</p>
  
  <p>When I use any column in the order by clause, (priority, start_date, completion_date...) it works perfectly, but whenever I put created_on it gives me the following error:</p>
  
  <pre><code>Unknown column 'created_on' in 'order clause'
  </code></pre>
  
  <p>Also, when trying <code>t</code>.<code>created_on</code> I keep having the same error:</p>
  
  <pre><code>Unknown column 't.created_on' in 'order clause'
  </code></pre>
  
  <p>In the fiddle I provided you can see the structure for Task table and the query I'm trying to run. I'd like to know why the union is not working with created_on column and how to make it work.</p>
  
  <p>Thanks in advance.</p>