#思路：
#①先对商品分组聚合求出购买数量，选出20个以上的部分；
#②再跟第一张表连接，选出质量50以下的部分即可。

select id, name, weight, total
from
(select goods_id, sum(count) as total
from trans
group by goods_id
having sum(count)>20) as tb1
left join 
goods as tb2
on tb1.goods_id=tb2.id 
where weight<50
order by id