HardSQL challenge #278WINDOW FUNCTIONGROUP BYPARTITION

Top Rated per Category

Find highest rated product in each category.

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

Solve This SQL Challenge

The task

Find highest rated product in each category.

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

`reviews` 180 rows

id INTEGER (PK) · product_id INTEGER · user_id INTEGER · rating INTEGER · comment TEXT · reviewed_at TEXT · helpful_votes INTEGER

First 3 rows of the reviews table
id	product_id	user_id	rating	comment	reviewed_at	helpful_votes
1	23	45	5	Works perfectly. Very happy with this purchase.	2022-09-02	40
2	101	68	5	Excellent build quality. Will buy again!	2023-01-05	96
3	40	39	1	Would give 6 stars if I could!	2023-05-15	17

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	avg_rating	rn
10	Magnetic Phone Mount Pro	Automotive	1599.08	495.71	411	SKU-000010	active	3.1	176	3.5	1
43	Deep Work (Newport)	Books	293.77	135.13	141	SKU-000043	active	4	314	5	1
86	Waterproof Rain Coat	Clothing	281.33	123.79	229	SKU-000086	active	3.6	448	5	1
49	Laptop Pro 15"	Electronics	452.25	140.2	167	SKU-000049	active	3.8	12	3.5	1
56	Raw Wildflower Honey 32oz	Food & Beverage	664.23	318.83	437	SKU-000056	active	3.2	82	5	1
6	Retinol Eye Cream 30ml	Health & Beauty	235.11	82.29	165	SKU-000006	active	3.7	463	3.5	1
108	Succulent Plant Set	Home & Garden	1990.16	616.95	107	SKU-000108	discontinued	4	224	4.5	1
91	Heavy Duty Stapler 70-Sheet	Office Supplies	206.29	111.4	57	SKU-000091	active	4.1	64	5	1
53	Sleeping Bag -10C	Sports & Outdoors	1098.92	417.59	463	SKU-000053	active	3.2	467	4	1
115	Catan Board Game	Toys	1273.17	522	124	SKU-000115	active	2.9	337	5	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
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
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

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. Join reviews on products.id = reviews.product_id.
3. Group the rows by products.id, so the result has one row for each value.
4. For each group, compute AVG(reviews.rating).
5. Number the rows with ROW_NUMBER(), ordered by AVG(reviews.rating) descending, restarting for each products.category.
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.