title: computed column with aggregate function
body: <p>I have seven columns in my table:
  hours, weight, status, total_hours, total_weight and percentage</p>
  
  <pre>
  total_weight = weight where status = 'X'
  total_hours = hours where status = 'X'
  percentage = total_hours/sum(weight)
  </pre>
  
  <p>sum(weight) is an aggregate function</p>
  
  <p>I would like to specify formula to generate the three computed columns. 
  What do i do?</p>