title: Getting difference between summs
body: <p>I have this mysql query</p>
  
  <pre><code>SELECT YEAR(date_entered), MONTHNAME(date_entered), SUM(amount)
  FROM opportunities
  GROUP BY YEAR(date_entered), MONTH(date_entered);
  </code></pre>
  
  <p>which brings back this data</p>
  
  <pre><code>"year(date_entered)","monthname(date_entered)","sum(amount)"
  "2009","March","9038714.77"
  "2009","April","2205819"
  "2009","May","9706852.6"
  </code></pre>
  
  <p>what i need to do is show differences? example march was 903, april was 220 so difference will be 680, and so on, so latest sum value minus precious months value?</p>
  
  <p>thanks in advance</p>