Good job! The last way to refer to another table in an INSERT, UPDATE, or DELETE statement is to use a subquery. Here is a DELETE statement that uses a subquery in the WHERE clause:

DELETE FROM product
WHERE id NOT IN (SELECT product_id FROM purchase)
  AND delivered_timestamp < CURRENT_TIMESTAMP - INTERVAL '3 years';

The subquery allows us to select the ID of all products that have appeared in an order (SELECT product_id FROM purchase). We're looking for products that have not been ordered (i.e., where the product ID is not in the list returned by the subquery) – hence the NOT IN operator. These are products that were not bought by anybody.

There's an additional condition in the WHERE clause:

delivered_timestamp < CURRENT_TIMESTAMP - INTERVAL '3 years';

This checks if each product's delivery date is more than three years old. Products not bought within the last three years are withdrawn.