选择最大值

Question

我很努力地找到一种方法来获取查询中每行的最大值。 我的查询看起来是这样的：

SELECT f.FID, f.name, f.description, t.TID, t.topic, p.timestamp, u.username, u.ID 
FROM forum f 
LEFT JOIN threads t ON t.FID=f.FID AND t.TID=(SELECT p.TID FROM posts p WHERE p.timestamp=(SELECT MAX(p.timestamp) FROM posts p)) 
LEFT JOIN posts p ON p.TID=t.TID 
LEFT JOIN users u ON u.ID=p.authorID 

一些简单的解释：每一个岗位被绑定到一个线程的TID，每个线程被绑定到一个论坛的FID。 我想要做的是从每个论坛获取最新帖子（时间戳的最高值），但我有点卡在这里。 它看起来如何在当前状态下的画面：

Answer 1

一种方式来做到这一点是有一个额外的加入到包含每TID最大时间戳值派生表：

SELECT f.FID, f.name, f.description, t.TID, t.topic, 
     p.timestamp, u.username, u.ID 
FROM forum f 
LEFT JOIN threads t ON t.FID = f.FID 
LEFT JOIN (
    SELECT TID, MAX(timestamp) AS max_timestamp 
    FROM posts 
    GROUP BY TID 
) AS x ON t.TID = x.TIP 
LEFT JOIN posts p ON p.TID = t.TID AND p.timestamp = x.max_timestamp 
LEFT JOIN users u ON u.ID = p.authorID 

Answer 2

首先所有我会建议使用内部加入。然后，我们可以确定每个主题或论坛的最后发布内容（请参阅lp）并将其用作过滤器。

---

最后发表从每个主题：

SELECT 
    f.FID, f.name, f.description, t.TID, t.topic, p.timestamp, u.username, u.ID 
FROM forum f 
INNER JOIN threads t ON t.FID=f.FID 
INNER JOIN posts p ON p.TID=t.TID 
INNER JOIN users u ON u.ID=p.authorID 
INNER JOIN (
    SELECT TID, MAX(timestamp) AS timestamp 
    FROM posts 
    GROUP BY TID 
) lp ON lp.TID = t.TID AND lp.timestamp = p.timestamp 

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最后发表从各个论坛：

SELECT 
    f.FID, f.name, f.description, t.TID, t.topic, p.timestamp, u.username, u.ID 
FROM forum f 
INNER JOIN threads t ON t.FID=f.FID 
INNER JOIN posts p ON p.TID=t.TID 
INNER JOIN users u ON u.ID=p.authorID 
INNER JOIN (
    SELECT ft.FID, MAX(fp.timestamp) AS timestamp 
    FROM posts fp 
    INNER JOIN threads ft ON fp.TID=ft.TID 
    GROUP BY ft.FID 
) lp ON lp.FID = f.FID AND lp.timestamp = p.timestamp