MediumSQL challenge #167JOIN

Products Without Reviews

Find products that have no reviews.

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 products that have no reviews.

A correct answer returns 27 rows with 10 columns: id, name, category, price, cost, stock, sku, status, rating, review_count.

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
7	Giant Stuffed Teddy Bear	Toys	1130.58	361.79	453	SKU-000007	active	3.1	419
9	Merino Wool Sweater	Clothing	1233.93	616.97	457	SKU-000009	active	2.8	163
13	Retinol Eye Cream 30ml	Health & Beauty	684.54	294.35	227	SKU-000013	discontinued	3.6	161
14	Play Kitchen Deluxe	Toys	1840.63	773.06	366	SKU-000014	active	5	40
19	Resistance Bands Set	Sports & Outdoors	1701.28	697.52	251	SKU-000019	active	3.8	239

First 5 of 27 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
Testing for missing values with IS NULL and IS NOT NULL
Keeping every row of the left table with LEFT JOIN, matched or not
Finding rows with no match using LEFT JOIN … IS NULL

Hint and approach

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

Show the hint

LEFT JOIN reviews

Show the step-by-step approach

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

Read from the products table.
LEFT JOIN reviews on products.id = reviews.product_id. Rows from the left side that have no match are kept, with NULL in the joined columns.
Keep only the rows where the joined reviews columns are NULL. Those are the rows that found no match in the join.
Return every column of products.

Model solution

Checking your session…

What to watch for

Anti join. Test a column of the joined table that can never be NULL in a real row, normally its primary key. Testing a nullable column also returns matched rows where that column happens to be empty.
LEFT JOIN. A WHERE condition on the right-hand table removes the unmatched rows and turns the LEFT JOIN back into an inner join. Conditions on that table go in the ON clause.
IS NULL. Comparing with = NULL or <> NULL never matches any row. NULL has to be tested with IS NULL or IS NOT NULL.