Month-to-Month Spending Drift :Compare Customer Spending Across Months with Orders Intermediate

          with monthly_spending as
  (select customerid,
          date_trunc('month', ordertimestamp)::date as order_month,
          sum(totalamount) as monthly_total
   from orders
   group by customerid,
            date_trunc('month', ordertimestamp)::date)
select customerid,
       order_month,
       monthly_total,
       lag(monthly_total) over (partition by customerid
                                order by order_month) as previous_month_total,
                               monthly_total - lag(monthly_total) over (partition by customerid
                                                                        order by order_month) as month_over_month_change
from monthly_spending
order by customerid,
         order_month;