[remywang]

> the word UNKNOWN itself isn't used in SQL

That leads to an even more confusing point, that some systems (at least SQLite) overloads NULL to mean UNKNOWN, for example `SELECT 1 WHERE NULL = (NULL = NULL)`.

And the dangerous thing about NULLs is not when they are explicitly used in the query as a literal (as I did for brevity), but when they appear in tables. It's perfectly reasonable to assume `SELECT COUNT( * ) FROM t;` should be the same as `SELECT COUNT( * ) from t WHERE t.x = t.x OR t.x <> t.x`, but they are not the same because the latter does not return NULL rows. This has lead to real query optimizer bugs. For more examples see this paper https://dl.acm.org/doi/10.14778/3551793.3551818