;with ach as
(
    select convert(varchar(6),date,112) as date,sum(qty) as qty
    from tb
    group by convert(varchar(6),date,112)
)

select a.date,cast(a.qty*100./nullif(b.qty,0) as decimal(6,2)) pe
from ach a join ach b on datediff(mm,b.date+'01',a.date+'01') = 1