title: Zend Framework 2: sql subquery
body: <p>In ZF1 it worked like this:</p>
  
  <pre><code> $selectColumns = array(
     '*',
     'orders_total' =&gt; "(".$db-&gt;select()-&gt;from("orders", array("COUNT(*)"))-&gt;where("orders.parent_id=mytable.id").")",
   );
   $select = $db-&gt;select()-&gt;from('mytable', $selectColumns);
  </code></pre>
  
  <p>How to do this in the ZF2? Thanks.</p>