Interview › Running total

SQL interview: running total with SUM() OVER

A running total keeps every row. GROUP BY would collapse them. That is the whole trick.

Prompt. For each order, show the customer’s spend so far, ordered by order_date.

Hint: SUM(total_amount) OVER (PARTITION BY customer_id ORDER BY order_date, order_id)

Your T-SQL

SELECT order_id, customer_id, order_date, total_amount,
       SUM(total_amount) OVER (
         PARTITION BY customer_id
         ORDER BY order_date, order_id
         ROWS UNBOUNDED PRECEDING
       ) AS running_spend
FROM Orders;

ROWS UNBOUNDED PRECEDING is the honest default. RANGE can double-count same-day twins.

Then write it on a ticket

Monday is free. A colleague asks, you type, we mark it.

Clock in free