title: Sum columns in mysql then use the result in where clause
body: <p>I want to be able to do this:</p>
  
  <pre><code>SELECT dept.id, (invoices.col1 + invoices.col2 + invoices.col3) as sumTotal 
  FROM dept 
       INNER JOIN invoices ON invoices.id_dept = dept.id 
  WHERE sumTotal &gt; 10000
  </code></pre>
  
  <p>But I am getting an unknown column on using "sumTotal".</p>
  
  <p>Is this possible?</p>