MySQL 获取最大时间戳和关联键

Question

我有 2 个表，Posts 和 Replies。它们都包含插入数据时的 PostID 和 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做到这一点，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

我已经研究了 20 分钟了 - 我知道这是一个简单的解决方案。

澄清一下，PostID 是 Posts 的 PK，PostID 是 Replies 的 FK。我想知道，所有 PostID 上最近的 activity，按照 TimeStamp

的降序排列

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 获取最大时间戳和关联键

MySQL Get max TimeStamp and associated key

mysql

subquery

union-all