HardSQL challenge #339JOINGROUP BYANALYSIS

Revenue per Review

Find the products that generate the highest revenue per review.

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

Solve This SQL Challenge

The task

Find the products that generate the highest revenue per review.

A correct answer returns 92 rows with 2 columns: id, revenue_per_review.

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

`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	revenue_per_review
71	6215.54
3	5765.639999999999
5	5698.72
29	5391.5
89	5046.65

First 5 of 92 rows. 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
Querying the result of another query with a subquery in FROM
Sorting the result with ORDER 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

Aggregate revenue and review counts in separate subqueries, then join and divide

Show the step-by-step approach

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

Read from the products table.
Build a second inner query (s) to join to:
1. Read from the order_items table.
2. Group the rows by product_id, so the result has one row for each value.
3. For each group, compute SUM(subtotal).
Join that inner query on s.product_id = products.id.
Build a second inner query (r) to join to:
1. Read from the reviews table.
2. Group the rows by product_id, so the result has one row for each value.
3. For each group, compute COUNT(*).
Join that inner query on r.product_id = products.id.
Add the calculated column revenue_per_review: s.revenue * 1.0 / r.reviews.
Sort the result by revenue_per_review descending.

Model solution

Checking your session…

What to watch for

Derived table. Columns computed in the inner query are only visible outside under their alias, so every calculated column there needs a name.
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.