Product introduction analysis

Hi all,

I’m stuck with what I thought would have been easy.

I’m trying to find out which year/month a product was sold the first time out of a transaction log based on Year and Month (no date).

My thought was to create a pivot on the product where pivoted columns are Year-Month and detect when I have a first sales.

But I cannot find a way to do the latter.

Can anyone help?

Thanks a lot,

Raoul.