title: Union query in mongodb without using map/reduce
body: <p>How can I convert the following SQL query for mongo:</p>
  
  <pre class="lang-sql prettyprint-override"><code>SELECT column1 from table1
  UNION
  SELECT column1 from table2
  </code></pre>
  
  <p>I have up to 12 collections, one for each month:</p>
  
  <pre class="lang-none prettyprint-override"><code>data.2013.01
  data.2013.02
  ...
  ...
  data.2013.12
  </code></pre>
  
  <p>When plotting I'd like to show multiple months on graph at a time. That brings us to the question, how to create UNION in mongo.</p>