title: SQL query to group by and order by timestamp
body: <p>Hey guys, I've tried to get this right but I can't, maybe you can point me in the right direction</p>
  
  <p>I have 3 columns, 'url_id', 'timestamp' and 'o'.  I need to group by 'url_id' and sort by the most current timestamp.</p>
  
  <p>table "example"<br/></p>
  
  <pre><code>timestamp | url_id   |    o
  ----------------------------
  2000      | 1        |   50
  2007      | 1        |   70
  2011      | 1        |   90
  2001      | 2        |   20
  2006      | 2        |   50
  2009      | 2        |   40
  2011      | 2        |   10
  </code></pre>
  
  <p>'o' is the value at the end I want.  I was trying to do this with a subquery but kept getting the oldest value (tried order by, and had no luck).</p>
  
  <p>What am I doing wrong?  Is what I'm looking for actually require a subquery?</p>