[dspillett]

> There's no reason for NULL to not equal NULL

Only if you consider NULL to be an exact value, to have one meaning i.e. does not exist. In the mathematical set theory upon which relational database theory was built this is not the case.

NULL really represents “unknown” not just the subset that is “unknowable”/“does not exist” as which point not all NULLs are equal as they may represent entirely different things that you currently don't know - this is why NULL=<anything> == NULL (including NULL=NULL == NULL) and NULL≠<anything> == NULL.