Incremental count in SQL Server 2005 - sql-server-2005

Question

I am working with a Raiser's Edge database using SQL Server 2005. I have written SQL that will produce a temporary table containing details of direct debit instalments. Below is a small table containing the key variables for the question I'm going to ask, with some fictional data:
Donor_ID Instalment_ID Instalment_Date Amount
1234 1111 01/01/2011 £5.00
1234 1112 01/02/2011 £0.00
1234 1113 01/03/2011 £5.00
1234 1114 01/04/2011 £5.00
1234 1115 01/05/2011 £0.00
1234 1116 01/06/2011 £0.00
2345 2111 01/01/2011 £0.00
2345 2112 01/02/2011 £5.00
2345 2113 01/03/2011 £5.00
2345 2114 01/04/2011 £0.00
2345 2115 01/05/2011 £0.00
2345 2116 01/06/2011 £0.00
As you will see, some of the values in the Amount column are £0.00. This can occur when a donor has insufficient funds in their account, for example.
What I'd like to do is write a SQL query that will create a field containing an incremental count of consecutive £0.00 payments that resets after a non-£0.00 payment or after a change in Donor_ID. I have reproduced the above data below, with the field I'd like to see.
Donor_ID Instalment_ID Instalment_Date Amount New_Field
1234 1111 01/01/2011 £5.00
1234 1112 01/02/2011 £0.00 1
1234 1113 01/03/2011 £5.00
1234 1114 01/04/2011 £5.00
1234 1115 01/05/2011 £0.00 1
1234 1116 01/06/2011 £0.00 2
2345 2111 01/01/2011 £0.00 1
2345 2112 01/02/2011 £5.00
2345 2113 01/03/2011 £5.00
2345 2114 01/04/2011 £0.00 1
2345 2115 01/05/2011 £0.00 2
2345 2116 01/06/2011 £0.00 3
To help clarify what I'm looking for, I think what I'm looking to do would be similar to a winning streak field on a list of a football team's results. For example:
Opponent Score Winning_Streak
Arsenal 1-0 1
Liverpool 0-0
Swansea 3-1 1
Chelsea 2-1 2
Fulham 4-0 3
Stoke 0-0
Man Utd 1-3
Reading 2-1 1
I've considered various options, but have made no progress. Unless I've missed something obvious, I think that a solution more advanced than my current SQL programming level might be required.

Answer 1

If I am thinking about this problem correctly, I believe that you want a row number when the Amount is 0.00 pounds.
Select 0 as As InsufficientCount
, Donor_ID
, Installment_ID
, Amount
From [Table]
Where Amount > 0.00
Union
Select Row_Number() Over (Partition By Donor_ID Order By Installment_ID)
, Donor_ID
, Installment_ID
, Amount
From [Table]
Where Amount = 0.00
This union select should only give you 'ranks' where the Amount equals 0.

Answer 2

Am calling your new field streakAmount
ALTER TABLE instalments ADD streakAmount int NULL;
Then, to update the value:
UPDATE instalments
SET streakAmount =
(SELECT
COUNT(*)
FROM
instalments streak
WHERE
streak.donor_id = instalments.donor_id
AND
streak.instalment_date <= instalments.instalment_date
AND
(streak.instalment_date >
-- find previous instalment date, if any exists
COALESCE(
(
SELECT
MAX(instalment_date)
FROM
instalments prev
WHERE
prev.donor_id = instalments.donor_id
AND
prev.amount > 0
AND
prev.instalment_date < instalments.instalment_date
)
-- otherwise min date
, cast('1753-1-1' AS date))
)
)
WHERE
amount = 0;
http://sqlfiddle.com/#!6/a571f/18