mysql - 从join中获取最新记录

Question

我有以下查询：

SELECT f.name, MAX( cmt.created_at ) AS CommentDate, cmt.comment AS LastComment
FROM families f
LEFT JOIN comments cmt ON f.id = cmt.commentable_id
WHERE f.id =12212

我得到了评论的最后日期，但我没有得到最后的评论（根据那个日期），而是得到了第一个评论。那个怎么样？

Answer 1

这正是您的查询所要求的：您正在选择名称、最长评论日期，然后是加入的第一个评论（在这种情况下，恰好是第一个评论）。您需要在其中添加额外的逻辑才能获得最新评论。

不同的 SQL 风格有不同的方法来做到这一点，但以下应该适用于任何地方：

select    f.name,
          cmt.created_at as CommentDate,
          cmt.comment as LastComment
from      families f
left join comments cmt
on        cmt.commentable_id = f.id
left join comments latest_cmt
on        latest_cmt.commentable_id = f.id
and       latest_cmt.created_at > cmt.created_at
where     f.id = 12212
and       latest_cmt.id is null;

我们假设评论表有一个id字段作为它的主键。

Answer 2

试试这个方法：

SELECT f.name, cmt.created_at AS CommentDate, cmt.comment AS LastComment
FROM families f
LEFT JOIN comments cmt ON f.id = cmt.commentable_id
WHERE f.id =12212
and cmt.created_at = (
                        SELECT MAX( cmt.created_at ) 
                        FROM families f
                        LEFT JOIN comments cmt ON f.id = cmt.commentable_id
                        WHERE f.id =12212
                      )

或者

SELECT f.name, cmt.created_at AS CommentDate, cmt.comment AS LastComment
FROM families f
LEFT JOIN comments cmt ON f.id = cmt.commentable_id
WHERE f.id =12212
and cmt.created_at = (
                        SELECT MAX( created_at ) 
                        FROM comments
                        WHERE commentable_id =12212
                      )

Answer 3

假设您在 Sql-Server 上：您的查询不应该工作，因为没有分组。

我认为您需要的是TOP(1) 订购后 cmt.created_at DESC；

SELECT TOP (1) f.name, cmt.created_at AS CommentDate, cmt.comment AS LastComment
FROM families f
LEFT JOIN comments cmt ON f.id = cmt.commentable_id
WHERE f.id =12212
ORDER BY cmt.created_at DESC

对于 mysql，我认为您可以LIMIT改用TOP：

SELECT f.name, cmt.created_at AS CommentDate, cmt.comment AS LastComment
FROM families f
LEFT JOIN comments cmt ON f.id = cmt.commentable_id
WHERE f.id =12212
ORDER BY cmt.created_at DESC
LIMIT 1