Interview › Anti-join

SQL interview: customers with no orders (LEFT JOIN IS NULL)

“Employees without orders” is the same as “customers who never bought.”

Prompt. Return customers who have no row in Orders.

Hint: LEFT JOIN Orders, WHERE o.customer_id IS NULL

Your T-SQL

SELECT c.customer_id, c.email
FROM Customers c
LEFT JOIN Orders o ON o.customer_id = c.customer_id
WHERE o.customer_id IS NULL;

NOT EXISTS is often cleaner. NOT IN (subquery) dies if the subquery returns NULL.

Then write it on a ticket

Monday is free. A colleague asks, you type, we mark it.