title: add two results of two alias
body: <p>this is my query:</p>
  
  <p>in a query have many results that come from summations, these summations want to get the percentage among them, so I want to assign a variable, this solution a try since I can not add up the alias, I would like to add 
  2 AS a, 1 AS b of the form a + b AS result ... Same query header</p>
  
  <pre><code>Select a.*, SUM( a.val1 ) AS val1, SUM( b.val2 ) AS val2
  from quest1 a , quest2 b
  </code></pre>
  
  <p>I want to do this:</p>
  
  <pre><code>Select a.*, @N:= SUM( a.val1 ) AS val1,  @M:= SUM( b.val2 ) AS val2,
  (100*@N/@M) as percent
  from quest1 a , quest b
  </code></pre>