虽然麻烦但是挺好理解

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

主要就是子查询where g.weight<50

select t.goods_id as id,sum(t.count) as total from
trans as t left join goods as g
on t.goods_id = g.id
where  g.weight<50
group by t.goods_id

查出id和销售总量total，再用where筛选小于50的

结束子查询后在外围筛选中获取总数大于20的，也就是where tt.total>20