title: Using Column Alias in Same SELECT Clause
body: <p>Is there anyway to use a column alias in the same SELECT clause which it is being assigned? For example:</p>
  
  <pre><code>SELECT ord_id, candy_id, price, quantity, 
  price * quantity AS ext_cost, ext_cost * @tax_rate
  </code></pre>
  
  <p>returns an error because MySQL does not recognize "<code>ext_cost</code>" in the <code>ext_cost * @tax_rate</code> query. If it is not possible, is possible to return a table with everything listed in the first query without having to write something like this?</p>
  
  <pre><code>SELECT ord_id, candy_id, price, quantity, 
  price * quantity AS ext_cost, (price * quantity) * @tax_rate
  </code></pre>
  
  <p>Basically, I was just wondering if there was anyway to reuse <code>ext_cost</code> in the SELECT query.</p>