@Antc There's no such thing as "a meaningless join between two relations" or a join "not representing a semantically correct connection", so that quote is a terrible unhelpful & misleading choice of words. Every join & every query has a straightforward meaning in terms of the meaning of its parts. They do immediately before reasonably write of "[t]he erroneous inference of information by the user from the relations in a database". They similarly terribly phrase re semantics elsewhere too. "They proposed the notion of l-less joins to capture the intuition of correct joins in B query." "Informally, a query is sound if it always returns information that logically follows from the state and the constraints."