En tilgang er at bruge en korreleret underforespørgsel:
SELECT
CusName,
CusID,
Quantity,
(SELECT SUM(t2.Quantity) FROM yourTable t2
WHERE t2.CusID = t1.CusID AND t2.Date <= t1.Date) QuantityTotal,
Date
FROM yourTable t1
ORDER BY
CusID,
Date;