title: R : How to filter subset min of minutes in every hour?
body: <p>I'm still new to R. I can query selecting a first row of every hour by partition in SQL server management. I found a very big problem from my program it takes too long time with <code>sqlQuery()</code> function in R and more long on running my shiny web app so much. The one tested way to less take time is query of all data and prepare the data in R much faster.</p>
  
  <p>So here is my example data :</p>
  
  <pre><code>                    Date Val
  1    2017-06-11 04:00:02  83
  2    2017-06-11 04:01:02  77
  3    2017-06-11 04:03:01  78
  ..
  4    2017-05-11 05:00:03  83
  5    2017-05-11 05:01:02  95
  6    2017-05-11 05:02:02  10
  ..
  7    2017-05-11 06:01:00  29
  8    2017-05-11 06:02:02  39
  9    2017-05-11 06:03:03  85
  10   2017-05-11 06:04:02  71
  ..
  11   2017-05-11 07:05:02  33
  12   2017-05-11 07:06:02  14
  .. (many rows)
  13   2017-05-12 00:01:01  20
  14   2017-05-12 00:03:01  45
  </code></pre>
  
  <p>Sometimes there are missing rows during an hour. My problem is how to filter only a min minute row of every hour.</p>
  
  <p>My example output :</p>
  
  <pre><code>                    Date Val
  1    2017-05-11 04:00:02  83
  4    2017-05-11 05:00:03  83
  7    2017-05-11 06:01:00  29
  11   2017-05-11 07:05:02  33
  13   2017-05-12 00:01:01  20
  </code></pre>
  
  <p>Any idea please help. Thank you so much.</p>