title: sql select average from distinct column of table
body: <p>In my table I have some colms like this, (beside another cols)</p>
  
  <pre><code>col1 | col2
  s1   |  5
  s1   |  5
  s2   |  3
  s2   |  3
  s2   |  3
  s3   |  5
  s3   |  5
  s4   |  7
  </code></pre>
  
  <p>I want to have average of <strong>ALL</strong> col2 over Distinct col1.
  (5+3+5+7)/4=5</p>