title: SQL to get a difference in value for the same column but for different dates within the same table
body: <p>How do I get a difference in value of a particular column for a particular date to the value for the same column for the previous date? </p>
  
  <pre><code>Current Table:
  Date          Qty
  8-Jan-17      100
  9-Jan-17      120
  10-Jan-17     180
  
  Desired Output:
  
  Date         Diff Qty
  9-Jan-17      20
  10-Jan-17     60
  </code></pre>