Chris.Strolia-Davis (10/7/2009)
Excellent article.You mentioned not knowing a real world application of the CROSS JOIN.
In my experience, this is typically used for creating test data.
Sometimes you need to test data in all sorts of different configurations. By using a cross join, you can set up the different parameters and try all combinations.
Additionally, if you are trying to create bogus data for a test environment, this is one way of taking data from different parts of the real data and generating new data that is not actually real.
In many cases, this type of join is used on temporary or memory based tables in a batch since the data it produces often needs to go through additional transformation and filtering before it is useful.
I guess, technically, it isn't used in a "real world" application, but it is used for real world issues.
The real world applications for using CROSS JOINS are many and varied. Most of them revolve around the use of a Tally Table (Numbers Table) to do things like make a Tally CTE which in turn would be cross joined to a delimited column to do splits or used to generate contiguous dates, etc. When limited by Triangular self joins (about half a cross join but still uses CROSS JOIN), they can be used to generate "schedule pairs" and a whole lot more. And, you're also correct... they can be used to very quickly generate very large volumes of constrained randomized test data. It's not uncommon to see some of the frequent posters generate a million row test table to make their point about a performance problem/solution. Rog_os also pointed out a frequent use above.
--Jeff Moden
Change is inevitable... Change for the better is not.