title: yii: select sum using query builder
body: <p>I try to execute a simple query like this:</p>
  
  <pre><code>$tot = Yii::app()-&gt;db-&gt;createCommand()
                  -&gt;select('sum(field)')
                  -&gt;from('products')
                  -&gt;where('id = ' . $id)
                  -&gt;queryRow();
  </code></pre>
  
  <p>But $tot return me null value.</p>