HardSQL challenge #314JOINDATEGROUP BY

Top Products Last 3 Months

Find the products with the highest revenue in the last 3 months of the data, counting back from the most recent order date.

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

Solve This SQL Challenge

The task

Find the products with the highest revenue in the last 3 months of the data, counting back from the most recent order date.

A correct answer returns 37 rows with 2 columns: product_id, 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
id	user_id	total	discount	item_count	status	order_date	shipped_date	shipping_city	shipping_country
1	43	2638.11	0.25	3	cancelled	2024-04-24	NULL	Los Angeles	USA
2	8	2947	0.19	5	delivered	2024-03-15	2023-04-09	Chicago	USA
3	136	683.24	0.08	2	delivered	2023-02-15	2023-07-10	Los Angeles	USA

`order_items` 496 rows

id INTEGER (PK) · order_id INTEGER · product_id INTEGER · quantity INTEGER · unit_price REAL · subtotal REAL

First 3 rows of the order_items table
id	order_id	product_id	quantity	unit_price	subtotal
1	1	76	5	192.21	961.05
2	1	82	3	107.1	321.3
3	1	65	1	5.39	5.39

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
product_id	revenue
60	2664.92
51	2646.92
119	2147.35
72	2079.7200000000003
73	1908.04

First 5 of 37 rows. This is the output of the model solution on the practice database.

What you practise

Filtering rows with a WHERE condition
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
Combining rows from two tables on a matching key with an inner join
Using the result of one query inside another
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

Anchor the date filter on MAX(order_date), then GROUP BY product

Show the step-by-step approach

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

Read from the order_items table.
Join orders on order_items.order_id = orders.id.
Filter in WHERE: keep the rows where orders.order_date is at least DATE((SELECT MAX(order_date) FROM orders), '-3 month').
The subquery runs once and returns a single value. It works like this:
1. Read from the orders table.
2. Aggregate over all the rows that are left: compute MAX(order_date). With no GROUP BY, the result is a single row.
Group the rows by order_items.product_id, so the result has one row for each value.
For each group, compute SUM(order_items.subtotal).
Sort the result by revenue descending.

Model solution

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What to watch for

INNER JOIN. An inner join drops rows that have no partner on the other side, and it repeats a row once for every match, which inflates SUM and COUNT.
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.