mysql reference previous row

I'm trying to calculate churn using one m开发者_JAVA百科ysql query... I can do this in PHP, but it would be very helpful if mysql could do it.

Here's my query:

SELECT 
  bill_date_bil,
  account_number_bil,
 SUM(amount_bil)


FROM
  billing_bil 
WHERE account_number_bil = 20017
  AND type_bil <> 'CA' 
  AND type_bil <> 'CK' 
GROUP BY period (bill_date_bil)
ORDER BY bill_date_bil ASC

and here's what I get:

bill_date_bil  account_number_bil  sum(amount_bil)
-------------  ------------------  ---------------
2007-09-01                  20017        3498.5000
2007-10-01                  20017        8248.5000
2007-11-01                  20017        9886.0000
2007-12-01                  20017        9411.0000
2008-01-01                  20017        9411.0000
2008-02-01                  20017        9411.0000
2008-03-01                  20017       -6151.5000
2008-04-01                  20017        6118.5400
2008-05-01                  20017       11171.9800
2008-06-01                  20017        6434.3800
2008-07-01                  20017        6434.3800

I would like to have another column that gives me the difference between the current bill and the previous months bill.

Thoughts?

here's what I need:

bill_date_bil   account_number_bil  sum(amount_bil) churn
-------------   ------------------  --------------- 
9/1/2007    20017   3498.5                         0
10/1/2007   20017   8248.5                        4750
11/1/2007   20017   9886                           1637.5
12/1/2007   20017   9411                        -475
1/1/2008    20017   9411                           0
2/1/2008    20017   9411                           0
3/1/2008    20017   -6151.5                       -15562.5
4/1/2008    20017   6118.54                         12270.04
5/1/2008    20017   11171.98                            5053.44
6/1/2008    20017   6434.38                         -4737.6
7/1/2008    20017   6434.38                         0

Use this:

SET @sum_bill=0;
SELECT 
  bill_date_bil,
  account_number_bil,
 SUM(amount_bil),
@sum_bill := @sum_bill+SUM(amount_bil)


FROM
  billing_bil 
WHERE account_number_bil = 20017
  AND type_bil <> 'CA' 
  AND type_bil <> 'CK' 
GROUP BY period (bill_date_bil)
ORDER BY bill_date_bil ASC