Louis Davidson explains the unknown:
There is no simpler topic in relational comparisons than three valued logic. I am being mostly facetious about this, but in reality, it seems so simple that people don’t think about how a
NULLworks, and make mistakes all of the time. I was reading a post about this the other day on LinkedIn (which by no means could one ever find again!) where one of the comments chastised the author of the post for not understanding “the fundamentals” of relational theory. The original poster wasn’t completely right (and my post may not be completely perfect either, though I will back most of what I write with code.)In this post I want to point out a few of the key basics that one really should understand.
Click through for a primer on what NULL means and doesn’t mean. And by the time you’re done, I’d like to interest you in the power of 6th normal form, where you can effectively banish NULL into the abyss (at least until you join the bits back together).