The `NULL` value can be surprising until you get used to it. And after you get used to it. And it will forever be surprising. `NULL` is one of the things in relational databases that will never disappoint you with the number of surprises it can bring. Most of them are not pleasant, but it's easy to understand them only thinking about NULL as an absence of value.
History of NULL
NULL was introduced to relational databases as a way to describe missing or inapplicable information. It was a way to say, "I don't know" or "I don't care" or "I don't have this information". And It is a very good way to do so. Let's take a look at a simple example.