with temp as (
select 
    substr(SALES_DATE,1,7) as yyyymm
    ,sales_amt as amt
from 
    sales

)
select
    ym
    ,ym_amt
    ,sum(ym_amt) over(order by ym)
    ,sum(ym_amt) over(order by ym)
from
    (
     select
        yyyymm as ym
        ,sum(amt) as ym_amt
    from
        temp
    group by 
        yyyymm
    )
order by 
    1