title: Stata sum by group
body: <p>I got data like this:</p>
  
  <pre><code>tinh   huyen   xa   diaban   hoso   fee1   fee2   income
  1      1       4    12        13     100    200
  1      1       4    12        13     120    150
  1      1       5    11        14     130    140
  1      1       5    11        14     100    100
  1      1       6    14        20     130    140
  </code></pre>
  
  <p>So I'd like to sum fee1 and fee2 by (tinh, huyen, xa, diaban, hoso) to income column.</p>
  
  <p>Then result supposes like:</p>
  
  <pre><code>tinh   huyen   xa   diaban   hoso   fee1   fee2   income
  1      1       4    12        13     100    200   570(100 + 200 + 120 + 150)
  1      1       4    12        13     120    150   570(100 + 200 + 120 + 150)
  1      1       5    11        14     130    140   470(130 + 140 + 100 + 100)
  1      1       5    11        14     100    100   470(130 + 140 + 100 + 100)
  1      1       6    14        20     130    140   270(130 + 140)
  </code></pre>
  
  <p>Please help me for this expression.
  Any solution will be appreciated. Thanks.</p>