1. Think before you run it: how many rows does customers c LEFT JOIN orders o ON o.customer_id = c.id return?
2. Every customer with the id of each of their orders: the customer's name, then the order id. Customers without orders must be in the result.
3. The names of the categories that have no products.
4. Every product with its category name: the product name, then the category name. Mystery Box must be in the result.
5. The names of the departments that have no employees.