write a query to return the year, month, and sales columns, along with a $4$th column named prior$\_$month$\_$sales showing the sales from the prior month. There are only $12$ rows in the sales$\_$totals table, one for each month of $2020,$ so you will not need to group data or filter$/$ partition on region$\_$id or product$\_$id$.$ Please use a windowing fuction for this as shown in the class video.