title: how to minus negative values in sum?
body: <p>I am using <code>SUM()</code> function. But <code>SUM()</code> sums the negative value in the column. In a column if the value is positive then it should be added and for negative values should be substracted and not added as the <code>SUM()</code> </p>
  
  <pre><code>20.00
  20.00
  20.00
  20.00
  -20.00
  20.00
  20.00
  40.00
  20.00
  20.00
  20.00
  20.00
  20.00
  -20.00
  -20.00
  20.00
  </code></pre>
  
  <p>sum() should return 220 and not 440.
  Is returning 440.</p>