UNION clause allows obtaining a new view containing the tuples
from another two views or queries. This corresponds with the relational
algebra union operation but with some differences. In principle, to
execute a relational algebra union all the relations must have the same
schema, i.e. the same attributes. However, in Virtual DataPort if some
of the views have an attribute that the others do not have, this is
added to the resulting view (this corresponds to the relational
operation called extended union).
Furthermore, in this case the union includes repeated rows, that is, if
a row is in two tables, the tuple appears twice in the resulting view.
DISTINCT can be used in the
SELECT clause to avoid