Deriving Predicates from Predicates in SQL
The corresponding section in the theory book describes how predicates can be derived from predicates using (a) the logical connectives of the propositional calculus, such as AND, OR, and NOT, and (b) quantifiers, such as "there exists" (∃) and "for all" (∀). Here I examine how SQL's truth value, unknown, intrudes on those connectives and quantifiers.