Set operators
Set operators are used to connect different queries and produce a
resulting relation. Of course these set operations are only allowed if the
attributes match (eg. the domain or the number of attributes etc.). The
following set operators are used to join together sets of tuples. They might
already be known from the algebra of sets.
union
produces a union of different
sets
intersect
produces an intersection of different
sets.
minus
subtracts one set from another
one.
This query is used to find all names of customers who have a
magazine subscription and who have placed an advertising
in this magazine.