title: compare values of same column on same table
body: <p>This has got to be a simple question, just brain-dumbing right now...</p>
  
  <p>I've got one table, called 'foo'.  It's got two columns, 'id' and 'username'.</p>
  
  <p>The id is unique, but some of the usernames reference the same user; just one has a prefix on the username of 'xx_'.</p>
  
  <pre><code>ex:
  ID      USERNAME
  1       bob
  2       sam
  3       xx_bob
  </code></pre>
  
  <p>How can I figure out which of the users have a counterpart with the 'xx_' prefix?  And then which ones do not?</p>