In SQL, NULL is not a value. It is a state indicating that an item's value is unknown or nonexistent. It is not zero or blank or an “empty string” and it does not behave like any of these values. Few things in SQL lead to more confusion than NULL, and yet its workings shouldn't be hard to understand as long as you stick to the following simple definition: NULL means unknown.

Let me repeat that:

Keep this line in mind as you read through the rest of the article, and most of the seemingly illogical results you can get with NULL will practically explain themselves.