Sum of sales in last month

1st I would create a MonthYear field in the script like this

LOAD partner_id,

           order_date,

          Date(MonthStart(order_date), 'M-YYYY') as MonthYear,

          order_month_year,

          order_month,

          order_year,

          order_day,

          sales

FROM ....;

and then use this expression:

Sum({<MonthYear = {"$(=Date(AddMonths(Max(MonthYear), -1), 'M-YYYY'))"}>}sales)