MySQL JOIN 多个表，每个结果都有 LIMIT Last Table By DateTime

Question

我得到了一个有趣的。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条目，而不是用同一个用户将结果加倍，只是登录时间不同。

示例（这只是一个示例表）：

    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

下面会给我我正在寻找的结果，但看起来效率很低，而且真的很慢。有什么方法可以加快速度并提高效率？

我可以这样做

         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。

Answer 1

您可以像这样加入从子选择创建的“表”：

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