select MONTHLY as YM,
case when MONTHLY = min(MONTHLY) over()
     then 0
     else sum(NEW_MRR) over(order by MONTHLY Rows between Unbounded Preceding and 1 Preceding)
         +sum(EXPANSION_MRR) over(order by MONTHLY Rows between Unbounded Preceding and 1 Preceding)
         -sum(DOWNGRADE_MRR) over(order by MONTHLY Rows between Unbounded Preceding and 1 Preceding)
         -sum(CHURN_MRR) over(order by MONTHLY Rows between Unbounded Preceding and 1 Preceding) end as MRR
  from MRR_DATA
order by MONTHLY