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)