title: Datediff between 2 columns in same table
body: <p>I've got 2 date columns in my table <code>(start_date, end_date)</code>.</p>
  
  <p>I've tried <code>Datediff(day, start_date, end_date)</code>, but I was prompt with:</p>
  
  <blockquote>
    <p><strong>invalid column name</strong></p>
  </blockquote>
  
  <hr>
  
  <p><strong>How can I calculate the date difference between these 2 columns?</strong></p>