CROSS JOIN operation
A CROSS JOIN is a JOIN operation that produces the Cartesian product of two tables. Unlike other JOIN operators, it does not let you specify a join clause. You may, however, specify a WHERE clause in the SELECT statement.
Syntax
TableExpression CROSS JOIN { TableViewOrFunctionExpression | ( TableExpression ) }
Examples
The following SELECT statements are equivalent:
SELECT * FROM CITIES CROSS JOIN FLIGHTS
SELECT * FROM CITIES, FLIGHTS
The following SELECT statements are equivalent:
SELECT * FROM CITIES CROSS JOIN FLIGHTS
WHERE CITIES.AIRPORT = FLIGHTS.ORIG_AIRPORT
SELECT * FROM CITIES INNER JOIN FLIGHTS
ON CITIES.AIRPORT = FLIGHTS.ORIG_AIRPORT
The following example is more complex. The ON clause in this example is associated with the LEFT OUTER JOIN operation. Note that you can use parentheses around a JOIN operation.
SELECT * FROM CITIES LEFT OUTER JOIN
(FLIGHTS CROSS JOIN COUNTRIES)
ON CITIES.AIRPORT = FLIGHTS.ORIG_AIRPORT
WHERE COUNTRIES.COUNTRY_ISO_CODE = 'US'
A CROSS JOIN operation can be replaced with an INNER JOIN where the join clause always evaluates to true (for example, 1=1). It can also be replaced with a sub-query. So equivalent queries would be:
SELECT * FROM CITIES LEFT OUTER JOIN
FLIGHTS INNER JOIN COUNTRIES ON 1=1
ON CITIES.AIRPORT = FLIGHTS.ORIG_AIRPORT
WHERE COUNTRIES.COUNTRY_ISO_CODE = 'US'
SELECT * FROM CITIES LEFT OUTER JOIN
(SELECT * FROM FLIGHTS, COUNTRIES) S
ON CITIES.AIRPORT = S.ORIG_AIRPORT
WHERE S.COUNTRY_ISO_CODE = 'US'
