Thanks for replying.

The only link relating a '03' (Additional Cars) product to a '01' (Rental Cars) is attribute_id=1 (Related row_id) in the '03' additional cars product.

I thought about self joining...

Thanks to both of you. It's true, I've got to check with the business logic team to see exactly when I should roll up/down a month.

Klass-Jan, I've implemented your script...

New follow up question:

Found another problem, adding 1 day to the end dates fixed the problem of DATEDIFF(mm, 1-01-2001, 12-31-2002) = 11.

But this also created a new problem, for example...

Just DATEADD(dd, 1, end_date) and it works... Thanks Andy!

It seems kinda silly to have to do this, I wonder if this is something that the SQL team will improve for...

Try this:

select DATEDIFF(m, '6/01/2001', '5/31/2002')

This is exactly how I wrote it in my query:

DATEDIFF(mm, od.order_start_date_hj, od.order_end_date_hj)

and the "order_start_date / order_end_date" are in this format: 2002-10-31 00:00:00.000 and I'm using SQL...