The relational model, and by extension, the language SQL supports the notion of NULL marker. It is commonly used to indicate that some attribute is unknown or non applicable. NULL markers are a bit strange because they are not values per se. Hence, the predicate 1 = NULL is neither true nor false. Indeed, the inventor of the relational model, E. F. Codd, proposed a 3-value logic model: predicates are true, false or unknown. This lives on even today. Our entire civilization runs on database systems using an unintuitive 3-value logic. Isn’t that something!