Calculate Running Totals or Cumulative Sums with Window Functions

SELECT
  order_date,
  amount,
  SUM(amount) OVER (ORDER BY order_date) AS running_total_amount,
  SUM(amount) OVER (PARTITION BY customer_id ORDER BY order_date) AS customer_running_total
FROM
  orders
ORDER BY
  order_date;

How it works: This SQL snippet demonstrates how to calculate running totals (or cumulative sums) using window functions. The `SUM(amount) OVER (ORDER BY order_date)` expression computes a cumulative sum of `amount` values, ordered by `order_date`, for all rows up to the current row. This is useful for tracking progressive totals over time. The second `SUM(amount) OVER (PARTITION BY customer_id ORDER BY order_date)` example shows how to calculate a running total *per customer*, restarting the sum for each new `customer_id`. This pattern is invaluable for financial analysis, sales tracking, and various other time-series data reports.