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N038-E1 Tier 3 · Intermediate · easy ecommerce · Brightlane

Return the ID and amount of every order, plus the combined total amount across every order on each row

Part of Window Functions Introduction (OVER, PARTITION BY) in SQL

The problem

Brightlane's finance team wants every order's amount displayed alongside the company-wide revenue total for at-a-glance comparison.

Write a query to return the ID and amount of every order, plus the combined total amount across every order on each row.

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.

Output:

  • One row per order, with columns id, total_amount, and grand_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
FROM
  orders

The shape

SUM(total_amount) OVER () computes the company-wide revenue total once and copies that single value onto every output row, leaving each order's own id and total_amount untouched. The empty parentheses after OVER are what make the window the entire result set.

Clause by clause

  • SELECT id, total_amount returns each order's identifier and its individual amount, one row per order with no collapsing.
  • The window column is:
SUM(total_amount) OVER () AS grand_total

SUM runs across the rows in the window. OVER () with nothing inside it defines the window as every row in the result, so the sum is the combined total_amount across the whole orders table. The same scalar lands in grand_total on every row.

  • FROM orders reads every order. There is no WHERE because the grand total is over every order in the table.

Why this and not GROUP BY with a single total

A bare SELECT SUM(total_amount) FROM orders returns one row, the grand total alone. The finance team needs the grand total alongside every order's individual amount, not instead of it. OVER () is what keeps every row in the output while still attaching the aggregate. The empty window is the load-bearing piece.

You practiced SUM(...) OVER () — compute a single total across the entire result and replicate it onto every row, without collapsing the rows themselves.

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