MySQL联接每个表，每个结果带有LIMIT最后一个表，按DateTime [英] MySQL JOIN Multiple Tables With LIMIT Last Table By DateTime per result

问题描述

我有一个有趣的.我要加入3张桌子.我加入的最后一张表，我只想按日期查看最新的条目，而不想将相同用户的结果加倍，登录时间也不同.

I got an interesting one. I have 3 tables I am joining. The last table I joined, I want to only see the latest entry by date and not double up my results with the same user and just have different login times.

示例(这仅是示例表):

Example (this is a sample table only):

    SELECT a.user_id
       a.user_name,
       b.department,
       c.last_logon_date_time,
       c.computer_name
 FROM table1 a
 LEFT OUTER JOIN table2 b ON a.user_id = b.user_id
 LEFT OUTER JOIN table3 c ON c.user_id = c.user_id

以下内容将为我提供所需的结果，但是效率很低而且非常慢.有什么方法可以加快速度并提高效率吗?

Below will give me the results I am looking for but looks very inefficient and is really slow. Any way of speeding this up and making it more efficent?

我可以做到

         SELECT a.user_id
       a.user_name,
       b.department,
       (SELECT c.last_logon_datetime FROM table 3 c WHERE a.user_id = c.user_id ORDER BY c.last_logon_datetime DESC LIMIT 1) as last_logon_datetime,
       (SELECT c.computer_name FROM table 3 c WHERE a.user_id = c.user_id ORDER BY c.last_logon_datetime DESC LIMIT 1) as  computer_name
 FROM table1 a
 LEFT OUTER JOIN table2 b ON a.user_id = b.user_id

谢谢.

推荐答案

您可以联接到从子选择创建的表"中，如下所示:

You can JOIN to a "table" created from a subselect like this:

SELECT t1.user_id, t1.user_name, t2.department, t3.computer_name, t3.logon_date
FROM table1 AS t1
LEFT OUTER JOIN table2 AS t2 ON t1.user_id = t2.user_id
LEFT OUTER JOIN
(SELECT user_id, computer_name, MAX(last_logon_date) AS `logon_date` FROM table 3 GROUP BY computer_name) AS t3 ON t1.user_id = t3.user_id

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