title: mysql avoiding subqueries
body: <p>If I have a table with three columns</p>
  
  <pre>
  userid,eventid,entrytime
   </pre>
  
  <p>and values like 
  <pre></p>
  
  <code>userid,eventid,entrytime
    1      1      NULL
    2      1    2012-01-01
    1      2    2012-01-01
    2      2    2012-01-01
    3      1      NULL
    3      2      NULL
  </code></pre>
  
  <p>What is the best way of selecting userids where entrytime has always been NULL</p>