Question

我有2个表，Posts和Replies。它们都包含PostID，以及插入数据时的TimeStamp。

在单个查询中，我想知道两个表中的最新条目以及该条目的关联PostID。

我知道在MSSQL中我可以使用子查询来完成此操作，如下所示;

SELECT
    PostID,
    MAX(TimeStamp) as newTimeStamp
FROM (
    SELECT
        PostID as PostID
        ,TimeStamp as TimeStamp
    FROM
        Posts
    UNION ALL
    SELECT
        PostID as PostID
        ,TimeStamp as TimeStamp
    FROM
        Replies
)
GROUP BY
    PostID
ORDER BY
    newTimeStamp DESC

我现在已经20分钟了 - 我知道这是一个简单的解决方案。

为了澄清，PostID是Posts的PK，PostID是Replies的FK。我想知道，所有PostID上的最新活动，按TimeStamp降序排列

Answer 1

这个怎么样？

SELECT 
   , Posts.PostID
   , Posts.TimeStamp as LatestPost
   , Replies.TimeStamp as LatestReply
   , COALESCE(Replies.TimeStamp, Posts.TimeStamp) as LastActivity
FROM
    Posts 
LEFT OUTER JOIN
    Replies
        ON
    Replies.PostID = Posts.PostID
WHERE 
    Posts.TimeStamp = 
        (SELECT MAX(TimeStamp)
         From Posts sub 
         WHERE sub.PostID = Posts.PostID ) 
  AND
    (Replies.TimeStamp is NULL  
     OR Replies.TimeStamp =
        (SELECT MAX(TimeStamp)
         FROM Replies sub
         WHERE sub.PostID = Posts.PostID
        )
    )
ORDER BY 4 DESC

MySQL获取最大TimeStamp和相关密钥

1 个答案: