title: special ORDER BY in a Mysql request
body: <p>I have a mysql table defined like this:</p>
  <pre><code>posts(id_post, title, post_date)
  </code></pre>
  <p>post_date is null if the post is not published and is equal to the publication date if the post is published</p>
  <p>I would like to be able to list the posts in the following way: first display the posts with a post_date=null, then the posts classified by descending publication date.</p>
  <p>How to do ?</p>
  <p>Thanks :)</p>