title: Get average time between times in SQL
body: <p>I'm really clueless as how to do this.</p>
  
  <p>Say my table looks like so:</p>
  
  <pre><code>MESSAGES
  - id
  - date_posted
  - content
  </code></pre>
  
  <p>How do I get the average time between postings? (date_posted)</p>