title: To calculate difference between two datetimes in mysql
body: <p>I want the <strong>query</strong> to calculate the difference between two date time fields </p>
  
  <ul>
  <li><p>to calculate total time taken</p></li>
  <li><p>two date time fields are in same table
  as <strong>start_time and stop_time</strong></p></li>
  <li>And I want to calculate total start duration and stop duration</li>
  </ul>