How to filter distinct count

 and 

You are the first to come to mind on this question. It should be fairly simple for those of your ability levels . . .

I have a sales order number on my table with an order date and a posting date. A sales order can have multiple invoices attached to it but an invoice can only attach to a single sales order. I need to create a calculation to determine how many sales orders for the month have all associated invoices post on the same day as the order was placed. The part I am not sure about is how to make sure invoices associated with that sales order do not post on other days.

Any suggestions?

ANd, just for fun, my table lines are for items on the sales order so a single sales order can have multiple lines on the table.