SQL Practice Problem #016 - List emails of customers who ordered Gizmo products

Problem

Task

Return each customer email once for customers who ordered at least one product in the Gizmo category.

Schema

Table Schema

customers(id, name, email)
products(id, title, category)
orders(id, customer_id, product_id, ordered_at)

Input

Sample Data

customers

id	name	email
1	Alice	[email protected]
2	Bob	[email protected]

products

id	title	category
10	Mini Gizmo	Gizmo
11	Desk Mat	Office

orders

id	customer_id	product_id	ordered_at
401	1	10	2026-04-01
402	1	10	2026-04-05
403	2	11	2026-04-07

Output

Expected Output

email
[email protected]

Answer

Check Your Solution

Show Answer and Explanation

Correct Answer

SELECT DISTINCT c.email
FROM orders AS o
INNER JOIN customers AS c
  ON o.customer_id = c.id
INNER JOIN products AS p
  ON o.product_id = p.id
WHERE p.category = 'Gizmo';

Explanation

The orders table connects customers and products. DISTINCT is needed because the same customer can order more than one Gizmo product.

Common Mistakes

Joining customers directly to products without using orders.
Forgetting DISTINCT and returning duplicate customer emails.
Filtering c.category even though category belongs to products.

Concepts

Related Concepts

INNER JOIN DISTINCT Foreign Keys JOIN Three Tables

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