Okay. I think I figured out where the problem is. I also realized that I didn't need one of the clauses in one of the CASE statements, so I've removed that. It probably won't make a significant difference in the speed, but it will make it ever so slightly easier to understand.

;

WITH totals AS (

SELECT *, ABS(Amount) AS Amt, SUM(ABS(Amount)) OVER(PARTITION BY Customer, TransactionType ORDER BY Date ROWS UNBOUNDED PRECEDING ) AS tot

FROM #t1

)

, balances AS (

SELECT *, MIN(CASE WHEN TransactionType = 'Debit' THEN Date END) OVER(PARTITION BY Customer ORDER BY tot, TransactionType ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING) AS PaidDate,

LAG(TransactionType) OVER(PARTITION BY Customer ORDER BY tot, TransactionType) AS PrevTransactionType,

MAX(CASE WHEN TransactionType = 'Debit' THEN Date END) OVER(PARTITION BY Customer ORDER BY tot, TransactionType ROWS UNBOUNDED PRECEDING) AS LastPaid,

LAG(tot) OVER(PARTITION BY Customer ORDER BY tot, TransactionType) AS PrevTot

FROM totals

)

SELECT TransactionID, Customer,

CASE

WHEN PaidDate IS NOT NULL THEN 0

ELSE tot - PrevTot

END AS RemainingBalance,

CASE

WHEN PaidDate IS NOT NULL THEN PaidDate

WHEN PrevTransactionType = 'Debit' THEN LastPaid

END AS LastRedeemedDate

FROM balances

WHERE TransactionType = 'Credit'

ORDER BY Customer, Date

Drew