HardSQL challenge #371SELF JOINDATEAGGREGATION

Max Product Orders in 7-Day Window

Find products with highest order count in any rolling 7-day window.

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 products with highest order count in any rolling 7-day window.

A correct answer returns 492 rows with 2 columns: product_id, cnt.

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	cnt
3	4
23	4
29	4
38	4
22	2

First 5 of 492 rows. This is the output of the model solution on the practice database.

What you practise

Filtering a range of values with BETWEEN
Summarising many rows into one value with COUNT, SUM, AVG, MIN or MAX
Grouping by more than one column to get a row per combination
Combining rows from two tables on a matching key with an inner join
Joining a table to itself under two aliases
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

self join with date range

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, under the alias oi1.
Join orders again, under the alias o1, on oi1.order_id = o1.id.
Join order_items again, under the alias oi2, on oi1.product_id = oi2.product_id.
Join orders again, under the alias o2, on oi2.order_id = o2.id, and only where o2.order_date is between o1.order_date and 7 days after o1.order_date, both ends included.
Group the rows by oi1.product_id and o1.order_date, so the result has one row for each combination.
For each group, compute COUNT(*).
Sort the result by cnt descending.

Model solution

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What to watch for

Self join. Without a condition such as a.id < b.id, a self join pairs every row with itself and lists each pair twice.
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 on several columns. Each extra GROUP BY column splits the groups further. Grouping by a column that is unique per row gives back one group per row.