A subset relation can be established using the statement INTERSECT.
All cities are to be found that occur in the table 'customer' as well as in the table 'hotel'. Without the additional specification of ALL, an implicit DISTINCT is issued again.
SELECT city
FROM customer
WHERE zip < 50000
INTERSECT
SELECT city
FROM hotel
WHERE zip < 50000
Values repeatedly occurring in the subset are displayed by using the following statement. The result values only occur as often as the values from the two tables 'customer' and 'hotel' have a counterpart in the corresponding other table.
SELECT city
FROM customer
WHERE zip < 50000
INTERSECT ALL
SELECT city
FROM hotel
WHERE zip < 50000
