HardSQL challenge #365JOINGROUP BYAGGREGATION

Category Revenue Leaders

Find category contributing highest revenue overall.

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

Solve This SQL Challenge

The task

Find category contributing highest revenue overall.

A correct answer returns 1 row with 2 columns: category, revenue.

Tables used in this challenge

`products` 120 rows

id INTEGER (PK) · name TEXT · category TEXT · price REAL · cost REAL · stock INTEGER · sku TEXT · status TEXT · rating REAL · review_count INTEGER

First 3 rows of the products table
id	name	category	price	cost	stock	sku	status	rating	review_count
1	Play Kitchen Deluxe	Toys	1786.38	589.51	257	SKU-000001	active	4	57
2	Micro-Cut Paper Shredder 8-Sheet	Office Supplies	907.93	281.46	131	SKU-000002	active	2.5	447
3	4K Dual Dash Camera	Automotive	568.89	204.8	176	SKU-000003	active	4.1	29

`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
category	revenue
Office Supplies	47482.77

The complete result: 1 row. 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
Combining rows from two tables on a matching key with an inner join
Sorting the result with ORDER BY
Returning only the first rows with LIMIT

Hint and approach

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

Show the hint

JOIN products + order_items

Show the step-by-step approach

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

Read from the products table.
Join order_items on products.id = order_items.product_id.
Group the rows by products.category, so the result has one row for each value.
For each group, compute SUM(order_items.subtotal).
Sort the result by revenue descending.
Keep only the first row with LIMIT 1.

Model solution

Checking your session…

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.
LIMIT. LIMIT without ORDER BY returns an arbitrary set of rows. LIMIT is also dialect-specific: SQL Server uses TOP and Oracle uses FETCH FIRST.