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N040-M3 Tier 3 · Intermediate · medium ecommerce · Brightlane

Return the ID and amount of every order, plus the grand total of `total_amount` across every order and the running total accumulated through that order in order of `id`

Part of Aggregate Window Functions (SUM, AVG, COUNT OVER) in SQL

The problem

Brightlane's finance report shows each order's amount alongside both the overall total across every order and the running total accumulated up to that order.

Write a query to return the ID and amount of every order, plus the grand total of total_amount across every order and the running total accumulated through that order in order of id.

Assumptions:

  • The orders table has one row per order with an id and a total_amount.
  • The grand total is the combined total_amount across every order in the table. The same value should appear on every output row.
  • The running total at each row is the combined total_amount of every order whose id is less than or equal to that row's id. On the row with the largest id, the running total equals the grand total.

Output:

  • One row per order, with columns id, total_amount, grand_total, and running_total.
Schema · ecommerce 5 tables
categories
id integer
name text
parent_id? integer
products
id integer
name text
category_id integer
price numeric
stock_qty integer
attributes? jsonb
order_items
id integer
order_id integer
product_id integer
quantity integer
unit_price numeric
customers
id integer
name text
email text
city? text
country text
created_at timestamptz
is_active boolean
orders
id integer
customer_id integer
ordered_at timestamptz
status text
total_amount numeric
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Solution query
SELECT
  id,
  total_amount,
  SUM(total_amount) OVER () AS grand_total,
  SUM(total_amount) OVER (
    ORDER BY
      id
  ) AS running_total
FROM
  orders

The shape

Two windowed SUM expressions sit side by side and disagree on ORDER BY. SUM(total_amount) OVER () has no ORDER BY inside OVER, so it returns the grand total on every row. SUM(total_amount) OVER (ORDER BY id) has an ORDER BY, so it returns a running total. The presence or absence of ORDER BY inside OVER is the entire difference between the two columns.

Clause by clause

  • SELECT id, total_amount returns each order's identifier and amount. Both windowed sums are computed alongside.
  • SUM(total_amount) OVER () AS grand_total is the static window. The empty OVER () defines the window as every row in the table, with no ordering applied. The SUM collapses across that whole window and returns the same value, 126725.73, on every row. It is the same number a plain SELECT SUM(total_amount) FROM orders would return, just attached to every row instead of collapsing the table.
  • SUM(total_amount) OVER (ORDER BY id) AS running_total is the accumulating window. ORDER BY id inside OVER defines the window for each row as every row with an id less than or equal to the current one. The result grows row by row. On the last id, running_total equals grand_total, which is the visual confirmation that the accumulation has reached the full table.
  • FROM orders reads every order. Both windowed sums apply to the same row set; their results differ only because of what ORDER BY does inside OVER.

Why two windowed SUM expressions and not one grouped sum

A GROUP BY would collapse the rows. The report needs the per-order detail and the totals attached. Two windowed sums with different OVER clauses on the same row set is the canonical shape for that ask. The window mechanism is what makes both totals available on every order row at the same time.

You practiced two windows in one query — SUM OVER () for the static grand total and SUM OVER (ORDER BY ...) for the running total — same aggregate, two window definitions, two distinct meanings.

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