Why turkey has the same name as Turkey
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].