MySQL的順序由其它表的行

Question

我有這兩個表：

表一：
--- ID --- Name ---
表B：
--- ID --- ID_of_a --- Date ---

現在我要訂購的table a由行Date（降序）table b最新日期。例如： Table a有ID爲「1」和「2」的行。
Table b有這樣的行：{ID，ID_of_a，Date} {1,1，「2013-06-30」}，{2,1，「2013-07-01」}，{3，2，「2013 -07-02" }
從表ID的將是正確的順序：1 --- 2
查詢：

SELECT DISTINCT a.ID, a.Name FROM a, b WHERE a.ID=b.ID_of_a ORDER BY b.Date desc 

但是，這並不有時工作。

Answer 1

假設有表之間的關係1-1：

select a.* 
from a left outer join 
    b 
    on a.id = b.id 
order by b.date desc 

如果有b多行每個a，那麼你就需要一個group by：

select a.* 
from a left outer join 
    b 
    on a.id = b.id 
group by a.id 
order by max(b.date) desc 

Answer 2

如何

select a.name,a.id from a join (select id_of_a as id_a,max(dt) as max_dt from b group by id_of_a order by max_dt desc) e on e.id_a=a.id 

實施例

mysql> select * from a; 
+----+------+ 
| id | name | 
+----+------+ 
| 1 | a | 
| 2 | b | 
+----+------+ 
2 rows in set (0.00 sec) 

mysql> select * from b; 
+----+---------+------------+ 
| id | id_of_a | dt   | 
+----+---------+------------+ 
| 1 |  1 | 2013-06-20 | 
| 2 |  2 | 2013-07-01 | 
| 3 |  1 | 2013-07-02 | 
+----+---------+------------+ 
3 rows in set (0.00 sec) 

mysql> select a.name,a.id from a join (select id_of_a as id_a,max(dt) as max_dt from b group by id_of_a order by max_dt desc) e on e.id_a=a.id; 
+------+----+ 
| name | id | 
+------+----+ 
| a | 1 | 
| b | 2 | 
+------+----+ 
2 rows in set (0.00 sec) 

基本上你第一選擇每個id_of_a最大的日期，然後將其加入到a

我認爲它可以被重新寫入更有效地