DEE and DUM

There are two relations with no attributes called DEE and DUM

DUM : set of tuples = {}

DEE : set of tuples = { {} }. (there is a single tuple which is empty - i.e. there are no name,value pairs)

DEE is the identity for JOIN and Cartesian product: ∀R R⨯DEE = R

Under the correspondence between expressions of the relational algebra and expressions of the relational calculus (Codd's theorem), the algebra expressions DUM and DEE corresponds to the calculus expressions FALSE and TRUE respectively.