title: For each site, find unique number of users whose last visit was to that site
body: <p>given the table of the following:
  timestamp, user, site_id</p>
  
  <p>For each site, I want to find the number of unique users whose last visit was to that site.</p>
  
  <p>I found the number of unique users per site. 
  but does not correspond the last login and user can be unique in each site..</p>
  
  <pre><code>SELECT site_id, COUNT(DISTINCT user_id)
  FROM SWE
  GROUP BY site_id
  ORDER BY COUNT(*);
  </code></pre>
  
  <p>Please help. Thank you</p>