title: One random record from everyone month
body: <p>Say we have the following data frame:</p>
  
  <pre><code>    ShippedDate        OrderID  Subtotal
  1   1996-07-16 00:00:00 10248   440.00
  2   1996-07-10 00:00:00 10249   1863.40
  ...
  25  1996-08-06 00:00:00 10272   1456.00
  26  1996-08-12 00:00:00 10273   2142.40
  ...
  51  1996-09-11 00:00:00 10298   3127.00
  52  1996-09-13 00:00:00 10299   349.50
  ...
  78  1996-10-14 00:00:00 10325   1497.00
  79  1996-10-14 00:00:00 10326   982.00
  80  1996-10-14 00:00:00 10327   2262.50
  81  1996-10-17 00:00:00 10328   1168.00
  etc
  </code></pre>
  
  <ol>
  <li>How to choose a record (random) for each?
  For example </li>
  </ol>
  
  <pre>
  1   1996-07-16 00:00:00 10248   440.00
  2   1996-08-12 00:00:00 10273   2142.40
  3   1996-09-13 00:00:00 10299   349.50
  4   1996-10-14 00:00:00 10327   2262.50
  </pre>
  
  <ol start="2">
  <li>how to calculate the sum for each month?
  For example</li>
  </ol>
  
  <pre>
  
      1   1996-07 2303.4
      2   1996-08 3598.4
      3   1996-09 3476.5
      4   1996-10 5909.5
  </pre>