MediumSQL challenge #161HAVINGSUBQUERY

Find Products Rated Above the Overall Average

Return the products whose average review rating is higher than the average of all reviews.

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.

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The task

Find products with average rating above overall average.

A correct answer returns 43 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)
2	3
3	3
6	3.5
8	4
10	3.5

First 5 of 43 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

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
Filtering groups after aggregation with HAVING
Using the result of one query inside another

Hint and approach

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

Show the hint

Nested AVG

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).
Filter the groups in HAVING: keep those where AVG(rating) is greater than the value returned by a subquery.
The subquery in HAVING runs once and returns a single value. It works like this:
1. Read from the reviews table.
2. Aggregate over all the rows that are left: compute AVG(rating). With no GROUP BY, the result is a single row.

Model solution

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What to watch for

The subquery in HAVING is not correlated: it yields one number and is evaluated once. The benchmark is the mean of all reviews, which is not the same as the mean of the per-product averages.

HAVING. Conditions on single rows belong in WHERE and conditions on aggregates in HAVING. Putting a row filter in HAVING works, but it groups rows that are about to be thrown away.
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.
Subquery. A subquery compared with =, > or < must return exactly one row and one column. If it can return several rows, use IN or an aggregate.

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