title: Zend DB returning NULL value
body: <p>I have the following query that extends from Zend_DB_Table_Abstract</p>
  
  <pre><code>$select = $this-&gt;select()
              -&gt;from('expense_details',
               array('SUM(expense_details_amount) AS total'))
          -&gt;where('YEAR(expense_details_date) = ?', '2010')
              -&gt;where('MONTH(expense_details_date) = ?', '01')
              -&gt;where('expense_details_linkemail = ?', 'xxxx@gmail.com');
  </code></pre>
  
  <p>However it returning a NULL value despite its "equivalent" returning the desired value </p>
  
  <pre><code>SELECT SUM(expense_details_amount) AS total FROM expense_details
  WHERE 
  YEAR(expense_details_date) = '2010'                     
  AND MONTH(expense_details_date) = '01'
  AND expense_details_linkemail = 'xxxx@gmail.com'
  </code></pre>
  
  <p>Is my Zend_DB_Table construct above correct? </p>