with tmp as (
select
    *
    ,lag(new_mrr, 1) over(order by monthly)
    + lag(expansion_mrr, 1) over(order by monthly)
    - lag(downgrade_mrr, 1) over(order by monthly)
    - lag(churn_mrr, 1) over(order by monthly) 
    as "TMP_MRR"
from
    mrr_data
)
select
    monthly "YM"
    ,case
        when tmp_mrr is null then 0
        else sum(tmp_mrr) over(order by monthly rows between unbounded preceding and current row)
     end as "MRR"
from
    tmp
order by
    monthly
;