Why does NULL = NULL evaluate to false in SQL server

Question

In SQL server if you have nullParam=NULL in a where clause, it always evaluates to false. This is counterintuitive and has caused me many errors. I do understa

Answer 1

The confusion arises from the level of indirection (abstraction) that comes about from using NULL.

Going back to the "what's under the Christmas tree" analogy, "Unknown" describes the state of knowledge about what is in Box A.

So if you don't know what's in Box A, you say it's "Unknown", but that doesn't mean that "Unknown" is inside the box. Something other than unknown is in the box, possibly some kind of object, or possibly nothing is in the box.

Similarly, if you don't know what's in Box B, you can label your state of knowledge about the contents as being "Unknown".

So here's the kicker: Your state of knowledge about Box A is equal to your state of knowledge about Box B. (Your state of knowledge in both cases is "Unknown" or "I don't know what's in the Box".) But the contents of the boxes may or may not be equal.

Going back to SQL, ideally you should only be able to compare values when you know what they are. Unfortunately, the label that describes a lack of knowledge is stored in the cell itself, so we're tempted to use it as a value. But we should not use that as a value, because it would lead to "the content of Box A equals the content of Box B when we don't know what's in Box A and/or we don't know what's in Box B. (Logically, the implication "if I don't know what's in Box A and if I don't know what's in Box B, then what's in Box A = What's in Box B" is false.)

Yay, Dead Horse.