SQL Quest › SQL Interview Questions › Joins
LEFT JOIN: Watch the NULLs Appear
Support wants every customer listed with their orders — including the customers who have never ordered anything.
Use a LEFT JOIN from customers to orders and show name, membership, product, total. Order by name, then order_id.
The new idea, and it is the whole point of LEFT JOIN: rows on the left survive even when nothing on the right matches. Those rows come back with NULL in every column that came from orders.
Don't filter the NULLs out. Look at them. Scroll to Amy Taylor — she's there, with product and total empty. Six customers look like that. Change LEFT JOIN to JOIN and those six vanish without a word of warning; that silent disappearance is the single most common way a report ends up quietly wrong.
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: 46 rows — 40 real orders plus 6 customers carrying NULLs
Hint
SELECT, LEFT JOIN, NULL Handling, and open the hint there if you stall.Concepts
SELECT LEFT JOIN NULL Handling
Practise the topic: SQL practice questions · JOIN practice · NULL handling practice
In these company practice sets
Stripe · DoorDash · TikTok · LinkedIn
A SQL Quest challenge matched to patterns reported for these companies — not a question any of them has published.
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