title: Select N entries for each month value
body: <p>I have the following SQL table:</p>
  
  <pre><code>Id    DateTime              Speed
  1     2017-03-02 19:06:20   50
  1     2017-03-02 19:10:18   52
  1     2017-04-01 20:01:10   55
  2     2017-03-02 18:06:20   60
  2     2017-05-03 19:08:00   61
  3     2017-04-12 19:01:40   80
  3     2017-05-11 19:05:50   82
  3     2017-05-14 11:00:00   81
  </code></pre>
  
  <p>I want to select any 2 entries for each month, so that months <code>3</code>, <code>4</code> and <code>5</code> would have 2 observations.</p>
  
  <p>Can anyone help with how to do it?</p>
  
  <p>EXPECTED RESULT:</p>
  
  <pre><code>Id    DateTime              Speed
  1     2017-03-02 19:06:20   50
  1     2017-03-02 19:10:18   52
  1     2017-04-01 20:01:10   55
  3     2017-04-12 19:01:40   80
  2     2017-05-03 19:08:00   61
  3     2017-05-11 19:05:50   82
  </code></pre>