An operational methodolgY, founded on {predicate calculus}, dealing with descripitive expressions that are equivalent to the operations of relational algebra. {Codd' s reduction algorithm} can convert from {relational calculus} to relational algebra. Two forms of the relational calculus exist: the {tuple calculus} and the {domain calculus}. ["An Introduction To Database SYstems" (6th ed), C. J. Date, Addison WesleY]. (1998-10-05)