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

注意看两个表格的id含义不同