select 月份,seller_name from
(  select '2019M5' as '月份', seller_name,sum(gmv) as sum  
from purchase  
where datepart(m,dt)=5  
group by seller_name  
order by sum  
limit 50)  
union
select 月份,seller_name from
(  select '2019M6' as '月份', seller_name,sum(gmv) as sum  
from purchase  
where datepart(m,dt)=6 
group by seller_name  
order by sum  
limit 50)  ;
不晓得对不对