Over in [1] it was rediscovered that our documentation assumes the reader is familiar with NULL. It seems worthwhile to provide both an introduction to the topic and an overview of how this special value gets handled throughout the system.
Attached is a very rough draft attempting this, based on my own thoughts and those expressed by Tom in [1], which largely align with mine.
I'll flesh this out some more once I get support for the goal, content, and placement. On that point, NULL is a fundamental part of the SQL language and so having it be a section in a Chapter titled "SQL Language" seems to fit well, even if that falls into our tutorial. Framing this up as tutorial content won't be that hard, though I've skipped on examples and such pending feedback. It really doesn't fit as a top-level chapter under part II nor really under any of the other chapters there. The main issue with the tutorial is the forward references to concepts not yet discussed but problem points there can be addressed.
I do plan to remove the entity reference and place the content into query.sgml directly in the final version. It is just much easier to write an entire new section in its own file.
Reviewed the documentation update and it's quite extensive, but I think it's better to include some examples as well.