Relational Operators and Logical Operators
It prepares the ground for subsequent sections in which each specific relational operator is paired with its logical counterpart, such that, for example, r1 JOIN r2 denotes the relation representing the extension of the predicate p1 AND p2, the conjunction of the predicates for the operand relations. It follows that where we can find SQL counterparts of those relational operators, invocations of those counterparts will in turn represent extensions of the predicates.