獲取最後的成本爲每行

Question

我有2個表：

materials: 
     int id 
    text title 

    costs: 
    int id 
    int material_id 
    int cost 
datetime created_at 

數據：

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 

我想去年的成本資料。 這樣的：

id title cost 
1 Wood 210 

我試試這個：

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 

這：

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 

但成本有時是不正確的（！）。

一個值這個返回正確的成本：

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

現在我剛剛得到的材料清單，並獲得在PHP foreach循環費用（+1查詢的每一行）

請幫助我！

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