# 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'

Parent topic: JOIN operations