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，但是没有得到最后的评论（根据那个日期），我得到了第一个评论。那个怎么样？

Answer 1

尝试这种方式：

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

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 2

假设您使用的是Sql-Server：由于没有分组，您的查询不应该正常工作。

我认为您需要的是在订购后TOP(1) <{em> 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，我认为您可以使用LIMIT代替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

Answer 3

这正是您的查询所要求的：您正在选择名称，最大评论日期，然后是第一个加入的评论（在这种情况下，恰好是第一个评论）。你需要在那里加入额外的逻辑来获得最新的评论。

不同的SQL风格有不同的方法，但以下应该适用于所有地方：

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字段作为其主键。

从加入中获取最新记录

3 个答案: