with 
MON as (
select
cast(substr(MONTHLY,6,2) as integer)+1 as PRMON
,MONTHLY 
,NEW_MRR + EXPANSION_MRR -DOWNGRADE_MRR-CHURN_MRR as VAL
,sum(NEW_MRR + EXPANSION_MRR - DOWNGRADE_MRR - CHURN_MRR) over (order by MONTHLY) as MRR
from MRR_DATA
order by PRMON asc
) 
select
MD.MONTHLY as YM
,case when MON.MRR is NULL then 0 else MON.MRR end as MRR
from MRR_DATA MD
left join MON on cast(substr(MD.MONTHLY,6,2) as integer)=MON.PRMON
order by PRMON asc