Question

I am wondering whether CROSS JOIN can be safely replaced with INNER JOIN in any query when it is found.

Is an INNER JOIN without ON or USING exactly the same as CROSS JOIN? If yes, has the CROSS JOIN type been invented only to express intent better in a query?

An appendix to this question would be:

Can there be a difference using modern and widely used DBMSes when using CROSS JOIN ... WHERE x, INNER JOIN ... ON ( x ) or INNER JOIN ... WHERE ( x ) ?

Thank you.

Answer 1

In all modern databases all these constructs are optimized to the same plan.

Some databases (like SQL Server) require an ON condition after the INNER JOIN, so your third query just won’t parse there.

Visibility scope of the tables is in the JOIN order, so this query:

SELECT  *
FROM    s1
JOIN    s2
ON      s1.id IN (s2.id, s3.id)
CROSS JOIN
        s3

won’t parse, while this one:

SELECT  *
FROM    s2
CROSS JOIN
        s3
JOIN    s1
ON      s1.id IN (s2.id, s3.id)

will.