title: SQL delete rows after a select query with a limit
body: <p>I would like to delete all but the user's most recent 20 tests from a table that has multiple users in it.  </p>
  
  <p>attempt 1:</p>
  
  <pre><code>DELETE FROM tests WHERE EXISTS (SELECT * FROM tests WHERE user_id = 38 ORDER BY test_timestamp DESC LIMIT 20, 100);
  </code></pre>
  
  <p>attempt 2:</p>
  
  <pre><code>DELETE FROM tests WHERE user_id = 38 ORDER BY test_timestamp DESC LIMIT 20, 100;
  </code></pre>
  
  <p>Are there any changes that I can make either of the above samples to work or do I need to use PHP to setup a rowcount variable to work around my offset issue?</p>