I have a column "purchase date" that has dates and null/blank values by customer (have another column named "customer").
Dates range is 1/1/2014 to 2/13/2017.
Goal is to create a column that sums the "purchase date" count of purchases for those that are not null by month to get to month over month differences.
So 'Customer A' count of purchases in Jan 2017 is 25, and 20 in Dec 2016. I need the new column to calculate the difference of 5.
Your assistance is appreciated.