title: Average a column returned from a subquery
body: <p>Is is not possible to average the results from a subquery? </p>
  
  <p>Here's the query I'm fighting with:</p>
  
  <pre><code>SELECT AVG(
      SELECT SUM(`retail_subtotal`)
        FROM `order`
       WHERE `status` IN('O')
    GROUP BY `lead_id`
  );
  </code></pre>