select t1.MONTHLY as YM
    ,(t2.NEW_MRR + t2.EXPANSION_MRR - t2.DOWNGRADE_MRR - t2.CHURN_MRR)
        + (t1.NEW_MRR + t1.EXPANSION_MRR - t1.DOWNGRADE_MRR - t1.CHURN_MRR) as MRR
from MRR_DATA t1
    join (
        select * from MRR_DATA
        union all
        select 
            date(min(MONTHLY), '-1 months'),0,0,0,0
        from MRR_DATA
    ) t2
        on t1.MONTHLY = date(t2.MONTHLY, '+1 months')
order by t1.MONTHLY