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'