SQL Quest › SQL Interview Questions › Subqueries & CTEs
A Subquery That Returns a Set (IN)
Marketing is planning a US campaign and wants the customers who have ordered from the USA.
Show name and membership for those customers. Order by name.
The new idea: a subquery doesn't have to return one value — it can return a whole column of values, and IN checks membership against that list. WHERE customer_id IN (SELECT customer_id FROM orders WHERE country = 'USA') reads almost exactly like the English sentence.
Notice the shape difference from the previous challenge: > needs a single number, IN needs a set. Feeding a multi-row subquery to > is an error, and it's one of the first real error messages people hit.
Solve it in the browser editor →
Runs on SQLite in your browser, graded against the expected result, no signup. A wrong answer gets a diagnosis, not just "incorrect".
Schema
customers
| customer_id | name | signup_date | membership | total_orders | |
|---|---|---|---|---|---|
| 1 | John Smith | john.smith@email.com | 2023-01-15 | Gold | 15 |
| 2 | Emma Wilson | emma.wilson@email.com | 2023-03-20 | Silver | 8 |
| 3 | Michael Brown | michael.brown@email.com | 2023-02-10 | Gold | 12 |
orders
| order_id | customer_id | product | category | quantity | price | total | order_date | country | status |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 1 | Laptop Pro | Electronics | 1 | 1299.99 | 1299.99 | 2024-01-15 | USA | completed |
| 2 | 2 | Wireless Mouse | Electronics | 2 | 49.99 | 99.98 | 2024-01-16 | Canada | completed |
| 3 | 3 | Office Chair | Furniture | 1 | 349.99 | 349.99 | 2024-01-17 | USA | completed |
Expected output: 3 rows — Daniel Martinez, John Smith, Michael Brown
Hint
SELECT, Subquery, IN, and open the hint there if you stall.Concepts
SELECT Subquery IN WHERE
Practise the topic: SQL practice questions · CTE practice
Related questions
Where would this cost you points in an interview?
Ten questions, no signup: a Skillmap across nine SQL skills and the one to fix first.
Take the readiness test