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N018-M1 Tier 2 · Core SQL · medium ecommerce · Brightlane

Return the names of every customer with no order history

Part of LEFT JOIN and RIGHT JOIN in SQL

The problem

Brightlane's CRM team is running a reactivation campaign and needs to identify customers who have not yet placed an order.

Write a query to return the names of every customer with no order history.

Assumptions:

  • A customer with zero orders is one whose customer_id does not appear anywhere in the orders table.

Output:

  • One row per customer with no order history, with a single column name.
Schema · ecommerce 5 tables
categories
id integer
name text
parent_id? integer
products
id integer
name text
category_id integer
price numeric
stock_qty integer
attributes? jsonb
order_items
id integer
order_id integer
product_id integer
quantity integer
unit_price numeric
customers
id integer
name text
email text
city? text
country text
created_at timestamptz
is_active boolean
orders
id integer
customer_id integer
ordered_at timestamptz
status text
total_amount numeric
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Solution query
SELECT
  c.name
FROM
  customers c
  LEFT JOIN orders o ON c.id = o.customer_id
WHERE
  o.id IS NULL

The shape

The LEFT JOIN keeps every customer; the WHERE o.id IS NULL filter then keeps only the customers whose right-side columns came back as NULL — meaning the join found no matching order. The pair is the anti-join: surface every entity on the left that has no counterpart on the right.

Clause by clause

  • SELECT c.name returns just the customer name — the only column the reactivation campaign needs.
  • FROM customers c LEFT JOIN orders o ON c.id = o.customer_id pairs each customer with each of their orders. Customers with at least one order produce one row per order with real values in o.*. Customers with no orders produce a single row with NULL in every o.* column.
  • WHERE o.id IS NULL keeps only those single-row, unmatched customers. orders.id is the primary key of the orders table — a real order can never have a NULL id. So a NULL in o.id is unambiguous: the row was synthesised by the LEFT JOIN to preserve an unmatched left-side customer. The eight customers in the result — Omar Jensen, Mark Hayes, and so on — are exactly the eight who don't appear anywhere in orders.customer_id.

Why this and not WHERE o.customer_id IS NULL

Both checks work on this query because the right side of the join produces NULL in every right-table column simultaneously when there's no match. The convention is to check a primary key column (o.id) rather than the join key (o.customer_id), because the primary key is guaranteed non-nullable on real rows — there is no ambiguity between "the row is real and has NULL in this column" and "the row was synthesised by the outer join."

The trap

The trap is putting the existence check on the left-side key by accident. WHERE c.id IS NULL filters out everyone — customers.id is the primary key on the left table and is never NULL on a real row. The query runs, returns zero rows, and looks like a perfectly empty result set rather than a logic error. The IS NULL check belongs on the right side of a LEFT JOIN. The right-side column is the one that becomes NULL when no match exists; the left-side column always carries its real value.

You practiced the anti-join pattern: LEFT JOIN ... WHERE right_table.key IS NULL. The recurring shape any time the question is "which entities on the left have no counterpart on the right" — the LEFT JOIN preserves everyone, the IS NULL check on a right-side key keeps only the unmatched ones.

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