左加入最近的记录，如果存在的话

Question

我有table个人，我想拉取最近的事件记录，但可能没有。

所以我的问题是，即使我离开加入以确保我为每个人都获得了记录，我也只会带回有活动的人，因为我正在取消约会的最大值作为标准。

SELECT *
FROM tbl_people
LEFT JOIN tbl_events ON
    tbl_people.People_UID = tbl_events.People_UID
WHERE tbl_people.Active = 1
AND (
    SELECT MAX(Event_Date) FROM tbl_events
    )

Table 人们会有以下

People_UID

Table 事件将具有以下内容

Event_UID, People_UID, Event_Name, Event_Date

就像我说的，我想要的是这样的输出：

Jen, Had a Baby, 7/10/2015
Shirley
Susan
Megan, Had a Baby, 8/5/2014
etc.

我希望这是有道理的。

Answer 1

执行此操作的一种方法是在联接中使用相关子查询来检查是否不存在同一个人的任何事件，该事件具有较晚的日期：

SELECT *
FROM tbl_people p
LEFT JOIN tbl_events e 
    ON p.People_UID = e.People_UID 
    AND NOT EXISTS (
       SELECT 1 FROM tbl_Events 
       WHERE e.People_UID = People_UID 
         AND Event_Date > e.Event_Date
    )
WHERE p.Active = 1

虽然这可能不是最有效的解决方案；也许首先限制左连接集更好。（或者想想它的存在应该更好）。

给定样本数据集：

People_UID  Event_UID   People_UID  Event_Name          Event_Date
Jen         1           Jen         Had a Baby          2015-07-10
Jen         3           Jen         Bought a horse      2013-07-10
Shirley     NULL        NULL        NULL                NULL
Susan       NULL        NULL        NULL                NULL
Megan       2           Megan       Had a Baby          2014-08-05
Megan       4           Megan       Had another Baby    2015-08-05

这将是结果：

People_UID  Event_UID   People_UID  Event_Name          Event_Date
Jen         1           Jen         Had a Baby          2015-07-10
Shirley     NULL        NULL        NULL                NULL
Susan       NULL        NULL        NULL                NULL
Megan       4           Megan       Had another Baby    2015-08-05

使用联接可能如下所示：

SELECT *
FROM tbl_people p
LEFT JOIN (
    SELECT e.* 
    FROM tbl_events e 
    JOIN (
       SELECT PEOPLE_UID, MAX(EVENT_DATE) MDATE 
       FROM tbl_Events GROUP BY People_UID
    ) A ON A.MDATE = E.event_date AND e.People_UID = A.People_UID 
) B ON p.People_UID = b.People_UID 
WHERE p.Active = 1

Answer 2

select * from 
(  SELECT *, row_number() over (partition by tbl_people.name order by tbl_events.Event_Date desc) as rn 
     FROM tbl_people
     LEFT JOIN tbl_events 
       ON tbl_people.People_UID = tbl_events.People_UID
    WHERE tbl_people.Active = 1 ) t 
where t.rn = 1

Answer 3

这取决于数据库。

如果您使用 MySQL，您可以简单地将相关子查询放入外部连接的 on 子句中，如下所示：

select *
  from tbl_people p
  left join tbl_events_latest e
    on p.people_uid = e.people_uid
   and e.event_date =
       (select max(x.event_date) from tbl_events x where x.people_uid = e.people_uid)
 where p.active = 1

在其他数据库中，情况可能并非如此。例如，在 Oracle 中，您会得到一个错误。你也可以运行:

with tbl_events_latest as
 (select *
    from tbl_events e
   where event_date = ( select max(x.event_date)
                          from tbl_events x
                         where x.people_uid = e.people_uid ) )
select *
  from tbl_people p
  left join tbl_events_latest e
    on p.people_uid = e.people_uid
 where p.active = 1

Answer 4

如果是 Sql Server 你可以使用 OUTER APPLY:

SELECT *
FROM tbl_people p
OUTER APPLY(SELECT TOP 1 Event_Date 
            FROM tbl_events e
            WHERE p.People_UID = e.People_UID
            ORDER BY Event_Date DESC)oa
WHERE p.Active = 1

或者如果你想 select 约会的话：

SELECT *
FROM tbl_people p
OUTER APPLY(SELECT MAX(Event_Date) AS Event_Date 
            FROM tbl_events e
            WHERE p.People_UID = e.People_UID)oa
WHERE p.Active = 1

左加入最近的记录，如果存在的话

Left Join most recent record, if one exists

mysql

sql

join

relationship