mysql - MySQL按其他表行排序

Question

我有这两张桌子：

表 a：
--- ID --- Name ---
表 b：
--- ID --- ID_of_a --- Date ---

现在我想按最新日期table a的Date(desc) 对行进行排序。table b示例： Table a具有 ID 为“1”和“2”的行。
Table b有这样的行：{ID, ID_of_a, Date} {1, 1, "2013-06-30"}, {2, 1, "2013-07-01" }, {3, 2, "2013-07- 02" }
表 a 中 ID 的正确顺序是：1 --- 2
查询：

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

但这有时行不通。

Answer 1

假设表之间存在 1-1 关系：

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

b如果每个中有多行a，那么您需要一个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

怎么样

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

例子

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，您首先选择每个日期的最大日期id_of_a，然后将其加入a

我认为它可以重写以更有效地运行