select
    t.goods_id as id,
    g.name as name,
    g.weight as weight,
    sum(t.count) as total
from goods as g
    join trans as t on g.id = t.goods_id
where g.weight < 50 
group by t.goods_id, g.name, g.weight
having sum(t.count) > 20 
order by g.id
#where不能接聚合函数，where不能接聚合函数，where不能接聚合函数