title: How to get the average interval from a single datetime column?
body: <p>I have the following table in my mysql database.</p>
  
  <pre><code>user_id| department | comment_time
  -----------------------------------------
  ahnaf  |     OM1    | 2018-02-01 10:00:00
  muttaki|     OM2    | 2018-02-01 11:00:00
  sunny  |     OM3    | 2018-02-01 12:00:00
  ahnaf  |     OM1    | 2018-02-01 21:00:00
  john   |     OM2    | 2018-02-01 23:00:00
  </code></pre>
  
  <p>From this table i want to find out how often someone from a department comments.
  I am looking for an answer like: A person from OM1 comments every 5 minutes on average.</p>
  
  <p>I have no idea how to write a query for that.</p>