获取最后的成本为每行

Question

我有2个表：

materials: 
     int id 
    text title 

    costs: 
    int id 
    int material_id 
    int cost 
datetime created_at 

数据：

materials: 
1 Wood 
2 Steel 
3 ... 

costs: 
1,1,200,2015-07-02 
2,1,205,2015-07-03 
3,1,210,2015-07-04 

我想去年的成本资料。 这样的：

id title cost 
1 Wood 210 

我试试这个：

select materials.id, title, cost from materials 
left join (
    select * from costs 
    order by costs.id DESC 
) as costs on costs.material_id = materials.id 
group by materials.id 

这：

select materials.id, materials.title, c1.cost from costs c1 
    join (select material_id, max(id) as id from costs group by material_id) AS c2 
     on c1.id = c2.id 
left join materials on materials.id = c1.material_id 

但成本有时是不正确的（！）。

一个值这个返回正确的成本：

select cost, material_id 
    from costs 
    where material_id = 2 
     order by id DESC 
     limit 1 

现在我刚刚得到的材料清单，并获得在PHP foreach循环费用（+1查询的每一行）

请帮助我！

Answer 1

select m.id, m.title, c1.cost 
from materials m 
join costs c1 on c1.material_id = m.id 
join 
(
    select material_id, max(created_at) as dt 
    from costs 
    group by material_id 
) c2 on c2.material_id = m.id and c2.dt = c1.created_at