These are the dataset statistics. As you can see it runs over several years (from jan 2015 till may 2020)

Here is a sample of how my data looks like before transformation.

How can I transform my dataset to something like this ?
Sum of Sales per month per year.

I already tried the following:
- convert date to numerical (keep old attribute) => date_month
- aggregate sales to sum(sales)
- removed duplicates for date_month
- inner join on date_month

But this results in a sum of months over ALL the years instead. I only have 12 months now.

Any suggestions ?