In ICE #9 “join2” we left join our “join1” table and the cus…

In ICE #9 “join2” we left join our “join1” table and the customer invoices table using the InvoiceID variable. If instead we use a inner join, we find that 1 observation is excluded from the join1 table. This means that there must be a sales order from join1 where InvoiceID is null.