1

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条目,而不是用同一个用户将我的结果加倍,只是有不同的登录时间。

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,而且速度很慢。有什么方法可以加快速度并提高效率吗?

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

谢谢你。

4

1 回答 1

0

您可以加入从这样的子选择创建的“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
于 2013-01-30T23:40:23.643 回答