NOT IN Subqueries

While IN finds matching rows, NOT IN finds rows that don't match — this is extremely useful for finding missing or excluded data.

The Pattern

SELECT name FROM products
WHERE id NOT IN (SELECT DISTINCT product_id FROM order_items);

This finds products that have never been ordered.

💡 Real-World Uses

Question	NOT IN Pattern
Unsold products	<code>WHERE id NOT IN (SELECT product_id FROM order_items)</code>
Customers without orders	<code>WHERE id NOT IN (SELECT customer_id FROM orders)</code>
Employees without reviews	<code>WHERE id NOT IN (SELECT employee_id FROM reviews)</code>

⚠️ Warning: NULL Trap!

If the subquery returns any NULL values, NOT IN returns no rows at all! Always filter NULLs:

-- Safe version:
WHERE id NOT IN (SELECT product_id FROM order_items WHERE product_id IS NOT NULL)

Your Task

Find all products that have never been ordered. Show only the product name.