select MONTHLY as YM,
case when MONTHLY = min(MONTHLY) over()
     then 0
     else lag(NEW_MRR) over(order by MONTHLY)
         +lag(EXPANSION_MRR) over(order by MONTHLY)
         +lag(DOWNGRADE_MRR) over(order by MONTHLY)
         +lag(CHURN_MRR) over(order by MONTHLY) end as MRR
  from MRR_DATA
order by MONTHLY