具有两列的子查询答案

【问题标题】：Subquery with two columns具有两列的子查询
【发布时间】：2014-09-03 13:42:13
【问题描述】：

我有两张桌子，像这样：

a.id、a.created、a.b
b.id, b.name, b.created

我希望表 b 中的所有记录都按 a.created 时间戳排序。

我一直在尝试以下方法：

SELECT b.id, b.name FROM b JOIN a ON a.b = b.id ORDER BY b.created DESC

但它总是从 b 返回多条记录，而我真正想要的只是 b 的每一行匹配一个 a。 p>

我尝试使用 SELECT 子查询：

SELECT b.id, b.name, (SELECT a.created AS a_c FROM a WHERE
a.b = b.id) AS a_c FROM b ORDER BY a_c DESC

但我也希望能够提取 a.id。

有没有我可以使用的 JOIN 语句或者我应该使用两个子查询？这将是我的应用程序中非常重要的功能，所以我希望性能高于标准！

【问题讨论】：

你试过了吗？ SELECT DISTINCT b.id, b.name FROM b JOIN a ON a.b = b.id ORDER BY b.created DESC
如果你也想拉a.id，并且a有多个匹配的行，你想拉哪一个？
@Barmar 我想匹配a 的最新记录。例如ORDER BY a.created DESC LIMIT 1
您需要进行子选择并加入其中。

标签： mysql

【解决方案1】：

我最终只是以suggested in this answer 的身份加入表格。

SELECT b.id, b.name, a.id AS a_id FROM b JOIN a ON
a.id = (SELECT a.id FROM a WHERE a.b=b.id ORDER BY 
a.created DESC LIMIT 1)

【讨论】：

【解决方案2】：

select
   b.id
  ,b.name
  ,b.created as b_created
  ,a.created as a_created 
from b
left join a
join (
   select b, max(created) as max_creatd
   from a
   group by b
)a_min
   on a_min.b = a.b
  and a_min.max_creatd = a.created
   on a.b   = b.id

对于这些数据集（避免编写大量 DDL 的 SQL Server 语法）：

a as (select * from (values
     ('A1','20140701 23:11:59', 'B1')
    ,('A2','20140701 22:11:59', 'B1')
  )a(id,created,b)

和

b as ( select * from (values
    ('B1', 'Bob', '20140701 11:59:59')
   ,('B2', 'Bub', '20140701 10:59:59')
  )b(id,name,created)

返回以下内容：

id   name b_created         a_created
---- ---- ----------------- -----------------
B1   Bob  20140701 11:59:59 20140701 23:11:59
B2   Bub  20140701 10:59:59 NULL

(2 row(s) affected)

【讨论】：

【解决方案3】：

select b.id, b.name, a.id a_id, a.created
from b join (
    select b, max(a.created) max_created
    from a
    group by b
) t1 on t1.b = b.id
join a on a.b = b.id and a.created = t1.max_created
order by a.created desc

【讨论】：

【解决方案4】：

SELECT b.id AS bid, b.name, a.id AS aid, a.created
FROM b
JOIN (SELECT a1.id, a1.b, a1.created
      FROM a AS a1
      JOIN (SELECT id, MAX(created) AS maxcreate
            FROM a
            GROUP BY id) AS a2
      ON a1.id = a2.id AND a1.created = a2.maxcreate) AS a
ON b.id = a.b
ORDER BY a.created DESC

子查询获取a 中每个id 的最新创建日期的行。然后你加入这个b。

【讨论】：