title: Count unique values per day, if timestamp has hours
body: <p>I have dataset: </p>
  
  <pre><code>timestamp               event   user
  2020-04-28 20:07:55.503 log_in  john
  2020-04-28 20:08:01.996 log_out john
  2020-04-28 20:08:02.470 log_in  john
  2020-04-28 20:08:03.996 log_out john
  2020-04-28 20:08:05.729 log_failed  john
  2020-04-29 10:06:45.683 log_in  mark
  2020-04-29 10:08:58.299 password_change mark
  2020-04-30 14:19:24.921 log_in  jeff
  2020-04-30 14:20:31.266 log_out jeff
  2020-04-30 14:21:44.438 create_new_user jeff
  2020-04-30 14:22:44.455 create_new_user jeff
  </code></pre>
  
  <p>How to write a sql query to count all unique events per day. the unclear part for me is the presence of hours in timestamp. The desired result looks like this:</p>
  
  <pre><code>timestamp  count       
  2020-04-28 3 
  2020-04-29 2
  2020-04-30 3
  
  </code></pre>