Except that NULL is not the same as UNKNOWN! NULL is a data value (like integers) that can appear in data expressions (like NULL + 1) and comparisons (like NULL = 1) whereas UNKNOWN is a truth value that can appear in boolean/logical expressions constructed from logical connectives like AND, OR, NOT.

A data expression always evaluates to a data value, and usually whenever any part of the expression is NULL, the entire expression evaluates to NULL.

A comparison evaluates to a truth value, and usually when a comparison invovles a NULL it returns UNKNOWN. This leads to weird behaviors where both `SELECT 3 WHERE NULL = NULL;` and `SELECT 3 WHERE NULL <> NULL;` returns nothing (because the query engine does not output a row if the predicate returns UNKNOWN on it).

What you listed above only comes into play for boolean/logical connectives like AND, OR, NOT, and in that case we follow 3-valued logic.

And there's more annoying corner cases when you deal with DISTINCT. The situation is so hopeless that SQLite has a whole table documenting divergent behaviors of NULL in different systems: https://www.sqlite.org/nulls.html