My goal is to demonstrate how many NEW customers are added each month. So if
a customer existed in January and also existed in February, they would only
be counted in the January total. If a customer appeared in March and not the
previous months then it would be counted only in March and not future months.
I am basically trying to determine the acceptance of a particular service.

I have two columns; one that has customer id and the other the month they
appeared. I know a long way to get what I want but am looking for a more
efficient method.