Question

A cross join performs a cartesian product on the tuples of the two sets.

SELECT * FROM Table1 CROSS JOIN Table2 

Which circumstances render such an SQL operation particularly useful?

Answer 1

If you have a ‘grid’ that you want to populate completely, like size and color information for a particular article of clothing:

select      size,     color from     sizes CROSS JOIN colors 

Maybe you want a table that contains a row for every minute in the day, and you want to use it to verify that a procedure has executed each minute, so you might cross three tables:

select     hour,     minute from     hours CROSS JOIN minutes 

Or you have a set of standard report specs that you want to apply to every month in the year:

select     specId,     month from     reports CROSS JOIN months 

The problem with maintaining these as views is that in most cases, you don’t want a complete product, particularly with respect to clothes. You can add MINUS logic to the query to remove certain combinations that you don’t carry, but you might find it easier to populate a table some other way and not use a Cartesian product.

Also, you might end up trying the cross join on tables that have perhaps a few more rows than you thought, or perhaps your WHERE clause was partially or completely missing. In that case, your DBA will notify you promptly of the omission. Usually he or she will not be happy.