> In SQL, NULL<>NULL yields false. No, NULL <> NULL yields UNKNOWN. That's why NULL <> NULL and NOT (NULL <> NULL) behave the same: they have the same value.

> In SQL, NULL<>NULL yields false. No, NULL <> NULL yields UNKNOWN. That's why NULL <> NULL and NOT (NULL <> NULL) behave the same: they have the same value. UNKNOWN is a first-order truth value in ternary logic. The key is that in a WHERE clause, a record is only returned if the WHERE clause evaluates to TRUE. Not TRUE or UNKNOWN. TRUE.