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N057-M1 Tier 4 · Advanced · medium ecommerce · Brightlane

Return each calendar quarter, the number of `orders` placed in that quarter, the total `orders` revenue, and the average value per order

Part of Grouping by Date Periods in SQL

The problem

Scenario: Brightlane's CFO requires a quarterly business review summarizing order activity for each fiscal quarter.

Task: Write a query to return each calendar quarter, the number of orders placed in that quarter, the total orders revenue, and the average value per order.

Assumptions:

  • The orders table holds one row per placed order, with the placement timestamp stored in ordered_at and the order amount stored in total_amount.
  • A calendar quarter is identified by its first day and covers every order placed within that quarter.

Output:

  • One row per calendar quarter present in the data.
  • Columns in this order: quarter_start (the first day of the calendar quarter), order_count, total_revenue, avg_order_value.
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
  DATE_TRUNC('quarter', ordered_at)::date AS quarter_start,
  COUNT(*) AS order_count,
  SUM(total_amount) AS total_revenue,
  AVG(total_amount) AS avg_order_value
FROM
  orders
GROUP BY
  DATE_TRUNC('quarter', ordered_at)

The shape

date_trunc('quarter', ordered_at) collapses every order timestamp down to the first day of its calendar quarter, so all orders placed in Q1, Q2, Q3, or Q4 of a given year share a single grouping key. Three aggregates then run side by side on each quarter's rows: COUNT for the order volume, SUM for the total revenue, and AVG for the per-order average.

Clause by clause

  • SELECT date_trunc('quarter', ordered_at)::date AS quarter_start, COUNT(*) AS order_count, SUM(total_amount) AS total_revenue, AVG(total_amount) AS avg_order_value returns one row per quarter with four columns. The ::date cast normalises the truncated timestamp to a plain date. The three aggregates are independent: SUM adds the dollars, COUNT counts the orders, AVG averages the dollars across the same set of rows.
  • FROM orders reads every placed order. There is no WHERE; the CFO's quarterly review covers every quarter in the data.
  • GROUP BY date_trunc('quarter', ordered_at) repeats the truncation as the grouping key. An order from January and an order from March of the same year both truncate to that year's January 1 and land in the same Q1 group.

The trap

date_trunc('quarter', ...) snaps to the first day of the calendar quarter (January 1, April 1, July 1, October 1), not to the first day of a fiscal quarter offset from the calendar. If Brightlane's fiscal year started in a month other than January, the 'quarter' field would still produce calendar-quarter buckets, and the totals would not line up with what the finance team expects on the fiscal report. The prompt says calendar quarter, so calendar-quarter truncation is correct here. Any change to a fiscal definition requires date arithmetic to shift the timestamp before truncation, not a different field name on date_trunc.

You practiced reporting per-quarter totals by truncating timestamps with date_trunc('quarter', ...), so every order in a quarter collapses into a single row.

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