title: SQL-how to bring the value of first row to the second row
body: <p>I have a table with following values:</p>
  
  <pre><code>week_no     amt   amt_diff
  1           500    100
  2           600    300
  3           900    100
  4          1000   null
  </code></pre>
  
  <p>When I subtract <code>week2.amt-week1.amt</code> the difference is getting saved in the <code>amt_diff</code> column of <code>week_no=1</code>. But I want the result to be stored with the <code>week_no=2</code> record.</p>
  
  <p>Can anyone help me with the SQL?</p>