Okay, now let's do a quick review of NULLs.

A NULL indicates that a value is unknown or missing. You might see a NULL in the last_name field of a client table if you don't have the client's last name (because you forgot to ask!). You may have a NULL in the launch_date field of a product table if that product hasn't been launched.

A NULL in a product_price field doesn't mean that the product is free, just that we don't know the price.

We use the operators IS NULL and IS NOT NULL to check if a value is NULL. IS NULL works like this:

SELECT *
FROM product
WHERE category IS NULL;