with 
t_union as (
    select 
        * 
    from 
        MRR_DATA
    union
    select
        cast(date(min(MONTHLY), '-1 months') as varchar),
        0,
        0,
        0,
        0 
    from 
        MRR_DATA
),
t_union_calc as (
    select 
        MONTHLY,
        sum(NEW_MRR + EXPANSION_MRR - DOWNGRADE_MRR - CHURN_MRR) over(order by MONTHLY) as MRR
    from 
        t_union
)
select 
    cast(date(MONTHLY, '+1 months') as varchar) as YM,
    MRR 
from 
    t_union_calc
order by 
    YM;