title: SQL query for select distinct with most recent timestamp first
body: <p>I have a mysql table with three columns: username, location, timestamp. This is basically a log of user activity of what location they are in and the time that they were there.</p>
  
  <p>What I want to do is select a distinct username+location where only the most recent item (by timestamp) is provided.</p>
  
  <p>So say the table consists of:</p>
  
  <pre><code>tom roomone 2011-3-25 10:45:00
  tom roomtwo 2011-3-25 09:00:00
  tom roomtwo 2011-3-25 08:30:00
  pam roomone 3011-3-25 07:20:23
  </code></pre>
  
  <p>I would want only these to be selected:</p>
  
  <pre><code>tom roomone 2011-3-25 10:45:00
  tom roomtwo 2011-3-25 09:00:00
  </code></pre>