How do you compute a running total in SQL Server?

Short answer: Use SUM(Amount) OVER (ORDER BY OrderDate ROWS UNBOUNDED PRECEDING). Prefer ROWS over RANGE when you want physical cumulative sums with ties on the order key.

Sample solution

SELECT OrderId, OrderDate, Amount,
       SUM(Amount) OVER (
           ORDER BY OrderDate, OrderId
           ROWS UNBOUNDED PRECEDING
       ) AS RunningTotal
FROM Orders;

Mention PARTITION BY CustomerId for per-customer running totals.