Customers Without Orders SQL Query | SQL Interview Question

Problem statement

Prompt

Given the customers and orders tables below, write a query that returns the name of every customer who has never placed an order.

Each customer should appear at most once in the result.
Only the customer name should be returned — no other columns.
orders.customer_id may be NULL; such orders do not belong to any customer and must not rescue anyone.

Example

customers		orders
id	name	id	customer_id
1	Alice	1	1
2	Bob	2	3
3	Carol	3	NULL
4	Dave

Expected result:

name
Bob
Dave

Constraints

customers.id is the primary key.
orders.customer_id references customers.id but is nullable.

Tips

This is an anti-join: keep rows from the left side that have no match on the right.
LEFT JOIN + WHERE orders.id IS NULL and NOT EXISTS are the two idiomatic forms.
If you use a subquery with NOT IN, remember how NULL values behave inside NOT IN (…).

Schema

CREATE TABLE customers (
    id   INTEGER PRIMARY KEY,
    name TEXT NOT NULL
);

CREATE TABLE orders (
    id          INTEGER PRIMARY KEY,
    customer_id INTEGER REFERENCES customers(id)  -- NULL allowed
);

Sample data

INSERT INTO customers (id, name) VALUES
  (1, 'Alice'),
  (2, 'Bob'),
  (3, 'Carol'),
  (4, 'Dave');

INSERT INTO orders (id, customer_id) VALUES
  (1, 1),
  (2, 1),
  (3, 3),
  (4, 3);

Additional hidden fixtures are applied during validation to test edge cases.

Solution

One correct approach — try solving it yourself in the practice editor first, then compare. There is usually more than one valid solution.

-- Reference: one row per customer with no matching order.
SELECT c.name
FROM customers c
WHERE NOT EXISTS (
    SELECT 1 FROM orders o WHERE o.customer_id = c.id
)
ORDER BY c.id;

Key concepts

LEFT JOIN
NOT EXISTS
Anti-join
NULL filtering

Customers Without Orders