title: Group by day of week and hour of day in SQL
body: <p>I have a column with filled with time stamps</p>
  
  <p>eg.    </p>
  
  <pre><code>               col1
  2017-01-01 10:30:00
  2017-01-02 08:10:00
  ...
  </code></pre>
  
  <p>I want to group by day of week then further group by hour of day.</p>
  
  <p>The output would look like </p>
  
  <pre><code>day  hour  count
    1     1      5
          2     12
    2     1     10
          5      3
    ...
  </code></pre>
  
  <p>How would I implement this in SQL? 
  I am using MySQL.</p>