D Generic name used to refer to any language that conforms to the principles laid down by The Third Manifesto.
D_UNION See disjoint union.
data An encoded representation of propositions, assumed by convention to be true ones.
data definition operator An operator that either (a) defines some database object, such as a base relvar or a view or a snapshot or a constraint, or (b) deletes (drops) or updates such a definition; in other words, an operator that updates the catalog. Note: Dropping a definition effectively causes the corresponding object to be dropped as well (at least as far as the user is concerned), and is usually described in such terms. For example, Tutorial D provides an operator it calls DROP CONSTRAINT (not "drop constraint ...