with t_a as (
select * from MRR_DATA
union
select '2021-12-01',0,0,0,0),
t_b as (
select MONTHLY,
sum(NEW_MRR + EXPANSION_MRR - DOWNGRADE_MRR - CHURN_MRR) over(order by MONTHLY) as MRR
from t_a
)
select cast(date(MONTHLY, '+1 months') as varchar) as YM , MRR from t_b
order by YM;