Good. Let's get started and refresh your memory about
NULL means that the value in question is unknown or missing. In a client table, you could have a
NULL last name because you forgot to ask your client for it.
product table, you could have a
NULL product launch date if it hasn't been introduced yet. A
NULL product price doesn't mean that the product is free, it simply means that the price is unknown.
We use the operators
IS NULL or
IS NOT NULL to check, respectively, if the value is missing or not, just like this:
WHERE category IS NULL