HardSQL challenge #264WINDOW FUNCTIONPARTITIONCALCULATION

Highest Margin per Category

Find product with highest profit margin in each category.

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

Solve This SQL Challenge

The task

Find product with highest profit margin in each category.

A correct answer returns 10 rows with 12 columns: id, name, category, price, cost, stock, sku, status, rating, review_count, margin, rn.

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

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
id	name	category	price	cost	stock	sku	status	rating	review_count	margin	rn
22	Back Seat Organizer	Automotive	1804.31	685.64	428	SKU-000022	active	3	131	1118.67	1
35	Clean Code (Martin)	Books	1891.09	699.7	50	SKU-000035	active	3.7	171	1191.3899999999999	1
18	Winter Parka Jacket	Clothing	1695.08	813.64	33	SKU-000018	active	3.5	328	881.4399999999999	1
84	Webcam HD 1080p	Electronics	1606.25	819.19	357	SKU-000084	active	3.6	441	787.06	1
68	Single Origin Coffee 500g	Food & Beverage	1991.75	657.28	195	SKU-000068	active	4.7	131	1334.47	1
5	Premium Beard Oil 60ml	Health & Beauty	1428.31	428.49	183	SKU-000005	active	5	305	999.8199999999999	1
108	Succulent Plant Set	Home & Garden	1990.16	616.95	107	SKU-000108	discontinued	4	224	1373.21	1
51	Ergonomic Desk Mat XL	Office Supplies	1839.35	717.35	267	SKU-000051	active	4.7	173	1122	1
65	Waterproof Hiking Boots	Sports & Outdoors	1769.01	566.08	164	SKU-000065	active	4.1	481	1202.9299999999998	1
1	Play Kitchen Deluxe	Toys	1786.38	589.51	257	SKU-000001	active	4	57	1196.8700000000001	1

The complete result: 10 rows. This is the output of the model solution on the practice database. The task does not fix a sort order, so your rows may come back in a different order.

What you practise

Filtering rows with a WHERE condition
Querying the result of another query with a subquery in FROM
Numbering rows with ROW_NUMBER() to pick one or a few per group
Restarting a window calculation for each group with PARTITION BY
Deriving a new column from an expression and naming it with an alias

Hint and approach

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

Show the hint

ROW_NUMBER partition

Show the step-by-step approach

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

Build an inner query first (a derived table):
1. Read from the products table.
2. Number the rows with ROW_NUMBER(), ordered by (price - cost) descending, restarting for each category.
3. Add the calculated column margin: price - cost.
Filter in WHERE: keep the rows where rn equals 1. The filter sits in the outer query because a window function cannot be tested in the WHERE clause of the query that computes it.

Model solution

Checking your session…

What to watch for

ROW_NUMBER(). ROW_NUMBER() breaks ties arbitrarily. When the ORDER BY inside OVER does not identify one row, the row that gets number 1 can change between runs.
PARTITION BY. PARTITION BY is not GROUP BY: it keeps every row and only decides which rows a window function looks at.
Derived table. Columns computed in the inner query are only visible outside under their alias, so every calculated column there needs a name.