MediumSQL challenge #153DATEGROUP BY

Calculate Monthly Revenue with strftime

Return revenue for every year-month in the orders table, oldest month first.

The challenge runs on the SQLabHub practice database (SQLite) and uses the orders table. You can solve it in the browser, with nothing to install.

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The task

Calculate total revenue per month using strftime to extract year-month.

A correct answer returns 36 rows with 2 columns: month, revenue.

Tables used in this challenge

orders 200 rows

id INTEGER (PK) · user_id INTEGER · total REAL · discount REAL · item_count INTEGER · status TEXT · order_date TEXT · shipped_date TEXT · shipping_city TEXT · shipping_country TEXT

First 3 rows of the orders table
iduser_idtotaldiscountitem_countstatusorder_dateshipped_dateshipping_cityshipping_country
1432638.110.253cancelled2024-04-24NULLLos AngelesUSA
2829470.195delivered2024-03-152023-04-09ChicagoUSA
3136683.240.082delivered2023-02-152023-07-10Los AngelesUSA

The first rows of each table are shown. See how all the tables relate on the database schema page.

Expected output

Expected output of the challenge
monthrevenue
2022-0123317.77
2022-023571.81
2022-039090.25
2022-0416409.3
2022-0511589.85

First 5 of 36 rows. This is the output of the model solution on the practice database.

What you practise

  • Summarising many rows into one value with COUNT, SUM, AVG, MIN or MAX
  • Grouping rows with GROUP BY to get one result row per group
  • Working with dates in SQLite: strftime, julianday and DATE
  • Sorting the result with ORDER BY

Hint and approach

Try the challenge first. Open these only when you need them.

Show the hint

Use strftime('%Y-%m', order_date) to group by month

Show the step-by-step approach

How the model solution is built, in the order the database evaluates it:

  1. Read from the orders table.
  2. Group the rows by month (the year-month of order_date), so the result has one row for each value.
  3. For each group, compute SUM(total).
  4. ROUND() trims the calculated figures to a fixed number of decimals.
  5. Sort the result by month.

Model solution

Checking your session…

What to watch for

Unlike a filtered revenue report, this sums every order whatever its status, cancelled and refunded included. ROUND(x, 2) trims the floating-point noise that adding REAL values leaves behind.

  • GROUP BY. Every selected column must either be in GROUP BY or sit inside an aggregate. SQLite accepts other columns and returns an arbitrary value for them; most databases raise an error.
  • Date functions. These date functions are SQLite syntax. MySQL uses DATE_FORMAT and DATEDIFF, PostgreSQL uses TO_CHAR, EXTRACT and date subtraction.
  • ORDER BY. Without ORDER BY the row order is not guaranteed, even if it looks stable. Rows that tie on the sort columns can come back in any order.
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