n Scalar variable of type V Variable whose permitted values are scalars from a specified
domain V.
n Tuple variable of type H Variable whose permitted values are tuples with a specified
tuple heading H.
n Relation variable (relvar) of type H Variable whose permitted values are relations
with a specified relation heading H.
n Database variable (dbvar) A named set of relvars. Every dbvar is subject to a set of
named integrity constraints and has an associated self-describing catalog.