Some cells in the tables are NULL. Find them, and make sure that your conditions do not lose them by mistake.

1. The customers with no email: name, then email.
2. The products with no category: name, then category_id.
3. Think before you run it: how many rows does SELECT name, category_id FROM products WHERE category_id <> 3; return? The table has 12 products.
4. Every product that is not in category 3 (Stickers), also the products with no category: name, then category_id.
5. The employees who have a manager: name, then manager_id.