title: Calculate the time difference between of two rows
body: <p>I have a table with column StartDate, I want to calculate the time difference between two consecutive record. </p>
  
  <p>Thanks. </p>
  
  <hr>
  
  <p>@ Mark Byers and  @ Yahia, I have request table as requestId, startdate  </p>
  
  <pre><code>requestId    startdate               
  1            2011-10-16 13:15:56
  2            2011-10-16 13:15:59
  3            2011-10-16 13:15:59
  4            2011-10-16 13:16:02
  5            2011-10-16 13:18:07
  </code></pre>
  
  <p>and i want to know what is the time difference between requestid 1 &amp; 2, 2 &amp; 3, 3 &amp; 4 and so on. i know i will need self join on table, but i am not getting correct on clause.</p>