聚合函数冲突

Question

我有两个表：

users: 
___________________________ 
|user_id  | username | 
|_______________|___________| 
| 1   | Dolly | 
| 2   | Didi  | 
|_______________|___________| 

forum: 
_____________________________________________________________ 
|match_static_id| comment | timpstamp   | user_id | 
|_______________|___________|______________________|__________| 
| 1   | Hi  | 2013-07-10 12:15:03 |  2 | 
| 1   | Hello | 2013-07-09 12:14:44 |  1 | 
|_______________|___________|______________________|__________| 

此查询工作正常，它使用只是THW forum表：

SELECT forum.match_static_id, 
count(forum.match_static_id) 'comments_no', max(forum.timestamp)'timestamp' 
FROM forum 
GROUP BY forum.match_static_id 
Order BY timestamp DESC 

但是，下面的查询使用两个表：

SELECT forum.match_static_id, 
count(forum.match_static_id) 'comments_no', max(forum.timestamp)'timestamp', users.username 
FROM forum 
INNER JOIN users on users.id = forum.user_id 
GROUP BY forum.match_static_id 

在这里我想得到的最大用户（时间戳），但我得到了错误的用户可以任何身体给我一个线索，请吗？ ORDER BY的时间戳降序

Answer 1

试试这个：

SELECT f1.match_static_id, 
    f2.comments_no, 
    f2.maxtimestamp, 
    users.username 
FROM forum AS f1 
INNER JOIN 
(
    SELECT match_static_id, 
    max(timestamp) maxtimestamp, 
    count(comment) AS comments_no 
    FROM Forum 
    GROUP BY match_static_id 
) AS f2 ON f1.match_static_id = f2.match_static_id 
     AND f1.timestamp = f2.maxtimestamp 
INNER JOIN users on users.user_id = f1.user_id; 

看到它在这里的行动：

• SQL Fiddle Demo