MySQL join table 像 concat 但只选择最新的行

Question

我遇到需要最新状态的情况。有两个 table，我将 table 简化为如下所示：

---

Table A
+------------+
|  a_id      |
|------------|
|  A1        |
|  A2        |
+------------+

Table B
+------------+-------------+------------------+------------------+
|  b_id      |  a_id       |  status          |  created_at      |
|------------+-------------+------------------+------------------|
|  B01       |  A1         |  something       |  2020-03-14      |
|  B02       |  A1         |  something else  |  2020-04-15      |
|  B03       |  A2         |  anything        |  2020-03-22      |
+------------+-------------+------------------+------------------+

---

我想显示来自 A 的 table 和 join table B 所以它会像这样显示：

---

+------------+--------------------+-----------------+
|  a.a_id    |  b.status          |  b.created_at   |
|------------+--------------------+-----------------|
|  A1        |  something else    |  2020-04-15     |
|  A2        |  anything          |  2020-03-22     |
+------------+--------------------+-----------------+

---

我将感谢 Mysql 查询或 codeigniter 查询生成器。谢谢

Answer 1

如果你是 运行 MySQL 8.0，你可以用 row_number():

select a.a_id, b.status, b.created_at
from tablea a
inner join (
    select 
        b.*, 
        row_number() over(partition by a_id order by created_at desc) rn
    from tableb b
) b on a.a_id = b.a_id and b.rn = 1

在早期版本中，一种选择是使用相关子查询进行过滤：

select a.a_id, b.status, b.created_at
from tablea a
inner join tableb b on a.a_id = b.a_id
where b.created_at = (
    select max(b1.created_at) from tableb b1 where b1.a_id = b.a_id
)

对于相关子查询解决方案的性能，请考虑 tableb(a_id, created_at) 上的索引。

Answer 2

1. 首先从 B table
中找到最新的 created_at 值

示例：SELECT MAX(created_at) FROM B GROUP BY a_id;

2. 然后在下面的查询中使用第一个结果，你会得到你想要的输出。

   FROM A 
   JOIN B ON A.a_id = B.a_id 
   WHERE B.created_at IN (SELECT MAX(created_at) FROM B GROUP BY a_id);