上面那个放进SQL有点问题，下面这一段可以跑出来：
select p1.月份,p1.seller_name from
( select '2019M5' as 月份, seller_name,sum(gmv) as sum
from purchase
where month(dt)=5
group by seller_name
order by sum
limit 50) p1
union
select p2.月份,p2.eller_name from
( select '2019M6' as 月份, seller_name,sum(gmv) as sum
from purchase
where month(dt)=6
group by seller_name
order by sum
limit 50) p2;
