检索最后一条记录各组中没有主键

Question

我有一个表来记录用户上次登录的详细

event_name userID value     created_at 
------------------------------------------------------------- 
last_login 12 2016-06-02 11:06:29 2016-06-02 11:06:29 
last_login 12 2016-06-02 10:16:11 2016-06-02 10:16:11  
last_login 14 2016-06-02 10:11:13 2016-06-02 10:11:13  
last_login 17 2016-05-02 10:16:29 2016-05-02 10:16:29  
last_login 14 2016-05-02 10:15:11 2016-06-02 10:15:11  

预期结果：

event_name userID value     created_at 
------------------------------------------------------------- 
last_login 12 2016-06-02 11:06:29 2016-06-02 11:06:29 
last_login 14 2016-06-02 10:11:13 2016-06-02 10:11:13  
last_login 17 2016-05-02 10:16:29 2016-05-02 10:16:29  

我要抢的用户ID此表信息组，我通过created_at命令并显示timesAgo fromat（例如1小时前）的值，但是我没有获得使用group的最新记录，例如，对于userID = 12，我得到的值= 2016-06-02 10:16： 11而不是值= 2016-06-02 11:06:29

Answer 1

如果我理解正确的话，你可以用NOT EXISTS()实现这一目标：

SELECT * FROM YourTable t 
WHERE NOT EXISTS(SELECT 1 FROM YourTable s 
       WHERE s.created_at > t.created_at 
        AND s.userID = t.userID) 

Answer 2

试试这个。

首先，我从created_at和用户ID获得最大，然后我加入让孔列

SELECT d.* FROM (
    SELECT 
     userID, MAX(created_at) AS created_at 
    FROM 
     tablename 
    GROUP BY userID) AS `keys` 
LEFT JOIN tablename d ON 
    d.userID = `keys`.userID 
    AND 
d.created_at = `keys`.created_at; 

样品

MariaDB []> SELECT d.* FROM (
    -> SELECT 
    ->  userID, MAX(created_at) AS created_at 
    -> FROM 
    ->  tablename 
    -> GROUP BY userID) AS `keys` 
    -> LEFT JOIN tablename d ON 
    -> d.userID = `keys`.userID 
    -> AND 
    -> d.created_at = `keys`.created_at; 
+--------+----------------------------+ 
| userID | created_at     | 
+--------+----------------------------+ 
|  1 | 0000-00-00 00:00:00.000000 | 
|  2 | 2016-06-03 13:16:07.000000 | 
+--------+----------------------------+ 
2 rows in set (0.00 sec) 

MariaDB []>