急求解决：统计当前月份之前一年收入占比(跨年),该如何处理

;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)) pefrom ach a join ach b on datediff(mm,b.date+'01',a.date+'01') = 1