with tmp as (
select * from mrr_data 
union all 
select date(max(monthly),'+1 months'),0,0,0,0 from mrr_data
),tbl as (
select
    monthly ,
    row_number() over(order by monthly) as row_num,
    NEW_MRR+EXPANSION_MRR-DOWNGRADE_MRR-CHURN_MRR as mrr
from tmp 
)
select 
    this.monthly as YM,
    ifnull(SUM(prev.mrr) OVER (ORDER BY this.monthly ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW),0) AS MRR 
from tbl as this 
inner join tbl as prev 
on this.row_num=prev.row_num+1 
order by this.monthly;