Question

我有三个表（MySQL）：

matches:
 ___________________________
|match_static_id| team_name |
|_______________|___________|
|  1            |  Italy    |
|  2            |  France   |
|_______________|___________|

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

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

以下查询工作正常（仅使用用户，论坛）：

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(match_static_id) 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
Order BY f2.maxtimestamp DESC

但是当我尝试使用此查询查询第三个表中的某些数据时：

SELECT  f1.match_static_id,
  f2.comments_no, 
  f2.maxtimestamp, 
  users.username,
  matches.team_name
FROM forum AS f1

INNER JOIN
(
  SELECT match_static_id, 
    max(timestamp) maxtimestamp,
    count(match_static_id) 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.id = f1.user_id
INNER JOIN matches on matches.match_static_id = f2.match_static_id
Order BY f2.maxtimestamp DESC

结果是重复的（每条记录都是重复的）我不知道为什么我看到一切都好所以如果有人有任何想法可以帮助我请帮助!!!

Answer 1

您可能不需要在内部select语句上进行连接。

SELECT  
  f1.match_static_id,
  count(f1.comment) as comments_no, 
  max(timestamp) as maxtimestamp, 
  users.username,
  matches.team_name
FROM 
    forum AS f1
INNER JOIN users u on u.id = f1.user_id
INNER JOIN matches m on m.static_id = f1.match_static_id
GROUP BY m.match_static_id
ORDER BY maxtimestamp DESC

Answer 2

我想，你想搜索最后一条评论，所以只需按功能分组

GROUP BY f1.match_static_id

SQL小提琴 - 非常有用

并且没有“users.id”列

与聚合函数问题的三个连接问题

2 个答案: