To be fair, in my opinion, you chose the wrong tool for the task. Why would you use a sum window function instead of a much simpler aggregate sum function (that does require a group by)?
Without the group by, you will be returning every row from the table, which isn't what the task calls for. You'd then have to do an additional row_number() or distinct or something to just get the unique rows per month per country.
Without the group by, you will be returning every row from the table, which isn't what the task calls for. You'd then have to do an additional row_number() or distinct or something to just get the unique rows per month per country.