Another thing to understand is how NULLs are handled in different contexts, using either the familiar two-valued logic or the more exotic three-valued logic. It

Another thing to understand is how NULLs are handled in different contexts, using either the familiar two-valued logic or the more exotic three-valued logic. It's kind of messy but really worth knowing if you're working with SQL. The wikipedia page actually gives a pretty good account of the issue: https://en.wikipedia.org/wiki/Null_%28SQL%29 https://en.wikipedia.org/wiki/Null_%28SQL%29