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N039-E1 Tier 3 · Intermediate · easy ecommerce · Brightlane

Return the ID and total amount of every order, plus a unique sequential number assigned in descending order of `total_amount`

Part of ROW_NUMBER, RANK, DENSE_RANK in SQL

The problem

Brightlane's fulfillment dashboard assigns every order a unique position based on order value for priority processing.

Write a query to return the ID and total amount of every order, plus a unique sequential number assigned in descending order of total_amount.

Assumptions:

  • The orders table has one row per order with an id and a total_amount.
  • The position is 1 for the highest total_amount and increments by 1 for each subsequent order.
  • Every order receives a different position; if two orders share the same total_amount, they still receive consecutive numbers in some order.

Output:

  • One row per order, with columns id, total_amount, and row_num.
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,
  ROW_NUMBER() OVER (
    ORDER BY
      total_amount DESC
  ) AS row_num
FROM
  orders

The shape

ROW_NUMBER() OVER (ORDER BY total_amount DESC) assigns each order a unique sequential integer in descending-total order. The largest order gets 1, the next-largest 2, and so on, with no gaps and no repeats, even when two orders share the same total.

Clause by clause

  • SELECT id, total_amount, ROW_NUMBER() OVER (ORDER BY total_amount DESC) AS row_num returns each order's ID, its amount, and a positional rank. The window's ORDER BY total_amount DESC defines the ordering inside which positions are assigned; the window has no PARTITION BY, so every order is in the same window and the positions run 1, 2, 3, ... straight through the table.
  • FROM orders reads the orders. Every order receives a row number; no row is dropped.

Why ROW_NUMBER and not RANK

If two orders tie on total_amount, ROW_NUMBER still hands out two distinct integers because every row gets its own number regardless of value. RANK would give them the same number and then skip the next integer. When the consumer needs a strict one-to-one mapping between orders and positions, ROW_NUMBER is the function that guarantees no repeats. The cost is that the tie-breaking between equal-amount orders is arbitrary in the absence of a secondary sort.

You practiced ROW_NUMBER() OVER (ORDER BY ...) — assign a unique sequential integer to every row; ties get consecutive numbers in an unspecified order.

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