Running Total Variation Query Time Optimization

Question

Hi all, 
I've been struggling with this query for a while. I need to set a customer specific running total for 10 million rows (reset for every customer). But every time the number goes negative, I need to set it as zero.
For example,
member no              amount            wallet
member1                 400                      400
member1                 -500                     0
member1                  200                    200
member2                  700                    700
member2                 -200                    500
Query:
DECLARE @member float
DECLARE @prev_member float
DECLARE @amount float
DECLARE @wallet float
DECLARE db_cursor CURSOR FOR  
SELECT [Member no], [Transaction Amount] 
FROM [wallet_master_3]
ORDER BY [Member No], [rownum]
FOR UPDATE
OPEN db_cursor
FETCH NEXT FROM db_cursor INTO @member, @amount
SET @prev_member = @member
set @wallet=0
WHILE @@FETCH_STATUS = 0
BEGIN   
       IF @prev_member <> @member set @wallet=0
       SET @wallet = @wallet + @amount
       IF @wallet < 0 SET @wallet = 0
       UPDATE [wallet_master_3] SET walletsize = @wallet
       WHERE CURRENT OF db_cursor
       set @prev_member=@member
       FETCH NEXT FROM db_cursor INTO @member, @amount
END   
CLOSE db_cursor   
DEALLOCATE db_cursor
I've tried using a cursor. In five minutes, it ran 17,000 rows but after running it for 15 hours, the code only manages to set the running total for 175,000 rows. I'm not exactly sure why. Is there a faster approach I can use? 
Thanks!

Answer 1

As an exercise a 'Quirky Update' may help you in this scenario. Try the below trick!
DECLARE @Wallet AS TABLE
MemberNo VARCHAR(10),
RowNum INT,
Amount INT,
Wallet INT
INSERT INTO @Wallet (MemberNo, RowNum, Amount) VALUES
('member1',1, 400),
('member1',2, -500),
('member1',3, 200),
('member2',1, 700),
('member2',2, -200)
DECLARE @RunTotal AS INT
UPDATE W1
SET
@RunTotal = W1.Wallet =
CASE
WHEN W1.RowNum = 1 THEN W1.Amount
WHEN @RunTotal + COALESCE(W1.Amount, W2.Amount) < 0 THEN 0
ELSE @RunTotal + COALESCE(W1.Amount, W2.Amount)
END
FROM @Wallet W1
LEFT OUTER JOIN @Wallet W2
ON W1.MemberNo = W2.MemberNo AND W2.RowNum = W1.RowNum - 1;
SELECT * FROM @Wallet;
RESULT
MemberNo RowNum Amount Wallet
member1 1 400 400
member1 2 -500 0
member1 3 200 200
member2 1 700 700
member2 2 -200 500
You can read more on 'Quirky Update' in below articles
Solving the Running Total and Ordinal Rank Problems - Jeff Moden
Robyn Page's SQL Server Cursor Workbench
NOTE: Please test it thoroughly before using in a production environment!
Krishnakumar S