HardSQL challenge #254GROUP BYORDER BYAGGREGATION

Rank Products by Their Average Review Rating

Return each reviewed product with its average rating, best rated first.

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

Solve This SQL Challenge

The task

Find products with highest average rating.

A correct answer returns 93 rows with 2 columns: product_id, avg_rating.

Tables used in this challenge

`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
product_id	avg_rating
115	5
98	5
91	5
86	5
74	5

First 5 of 93 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
Sorting the result with ORDER BY

Hint and approach

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

Show the hint

AVG + ORDER BY

Show the step-by-step approach

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

Read from the reviews table.
Group the rows by product_id, so the result has one row for each value.
For each group, compute AVG(rating).
Sort the result by avg_rating descending.

Model solution

Checking your session…

What to watch for

The average is calculated from the reviews table rather than read from products.rating. Products with no reviews are missing from the result, and a product with one 5-star review sorts to the top.

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.
ORDER BY. Without ORDER BY the row order is not guaranteed, even if it looks stable. Rows that tie on the sort columns can come back in any order.
Aggregate functions. Aggregates skip NULLs. AVG divides by the number of non-NULL values and COUNT(column) can be smaller than COUNT(*).