title: Order by `updated_at` and `created_at` even if they are NULL
body: <p>I have the following table:</p>
  
  <pre><code>CREATE TABLE `entries` (
    `id` int(11) NOT NULL AUTO_INCREMENT,
    `name` varchar(255) DEFAULT NULL,
    `created_at` datetime DEFAULT NULL,
    `updated_at` datetime DEFAULT NULL,
    PRIMARY KEY (`id`)
  ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
  </code></pre>
  
  <p>I want to sort the entries by both <code>updated_at</code> and <code>created_at</code>.</p>
  
  <p>But either of them could be <code>NULL</code>.</p>
  
  <p>My query:</p>
  
  <pre><code>SELECT *
  FROM `entries`
  ORDER BY `updated_at` DESC, `created_at` DESC, `id` DESC
  </code></pre>
  
  <p>But this will put every entry which has not been updated at the bottom of the results.</p>
  
  <p>Basically I need them to be sorted by the last date available. And if there is none available to be sorted by <code>id</code>.</p>