A table Orders(order_id, customer_id, order_date, amount) needs a query that returns each order alongside a running cumulative total of that customer's order amounts, ordered by order_date. Which SQL construct is specifically designed for this kind of calculation?

Window functions like SUM(amount) OVER (PARTITION BY customer_id ORDER BY order_date) are purpose-built for exactly this kind of running/cumulative calculation: PARTITION BY resets the running total separately for each customer, and ORDER BY within the OVER clause determines the row-by-row order in which values accumulate, all while preserving individual row-level detail in the output. A plain GROUP BY with SUM would collapse each customer into a single aggregated row, losing the per-order running total entirely. HAVING only filters already-grouped results. A correlated subquery without partitioning would need to be manually written per customer and is far less efficient and elegant. UNION ALL of separate queries doesn't naturally produce a running total either.