Question

我的LEFT JOIN查询工作正常，但在排序查询结果时遇到问题。

查询是

SELECT * 
FROM posts 
LEFT JOIN comments ON posts.postId = comments.commentId 
ORDER BY postDate DESC, commentDate DESC

相反，评论会在我列表的末尾拉出。

帖子

postId (Primary)    int(11)      AUTO_INCREMENT
postUserId          int(11)
postType            varchar(10)
postContent         text
postCaption         text
postState           varchar(10)
postVotes           int(11)
postDate            datetime

和评论

commentId (Primary) int(11)     AUTO_INCREMENT
commentUserId       int(11)
commentFor          varchar(20)
commentForId        int(11)
commentContent      text
commentState        varchar(20)
commentVotes        int(11)
commentDate         datetime

从查询中注释的顺序不正确从数据库中提取原始数据首先从评论中拉出

"1","1","post","1","this is one comment and the only one","published","1","2019-02-05 12:04:00"

这是来自帖子

"1","1","text","this is the first post",,"published","1","2019-02-05 12:02:00"
"2","1","text","this is the second post",,"published","1","2019-02-05 12:16:00"

所需结果应该像

"2","1","text","this is the second post",,"published","1","2019-02-05 12:16:00"
"1","1","post","1","this is one comment and the only one","published","1","2019-02-05 12:04:00"
"1","1","text","this is the first post",,"published","1","2019-02-05 12:02:00"

按日期/时间排序

Answer 1

先行

首先，您的加入条件有误。

因此，每个帖子和每个评论都有一个唯一的主键。这意味着您无法将postId与commentId结合使用，因为一个帖子可以包含许多评论..并且每个评论都需要一个不同的ID。

所以正确的条件是：

SELECT * 
FROM posts 
LEFT JOIN comments ON posts.postId = comments.commentForId 
ORDER BY postDate DESC, commentDate DESC

具有嵌套查询和顺序的UNION

现在，要组合和排序评论和帖子的结果，您需要使用嵌套查询和UNION。

示例：

select * from

(select postId as entryId, postContent as content, postDate as timestamp from posts
UNION
select commentId, commentContent, commentDate from comments)temp

ORDER BY temp.timestamp DESC;

有数据：

create table posts(
  postId int,
  postContent text,
  postDate datetime
  );

  create table comments(
    commentId int,
    commentForId int,
    commentContent text,
    commentDate datetime);

    insert into posts values (1, 'this is the first post', "2019-02-05 12:02:00");
    insert into posts values (2, 'this is the second post', "2019-02-05 12:16:00");

    insert into comments values (1, 1, 'this is the comment', "2019-02-05 12:04:00");

查看小提琴：https://www.db-fiddle.com/f/koRapukYqcSxWLNQ8hSA7Y/0

Answer 2

您的JOIN条件可疑：

posts.postId = comments.commentId

这表明名称完全不同的ID应该匹配。

大概是您打算的：

SELECT p.*, c.*   -- you should list the columns you want
FROM posts p LEFT JOIN
     comments c
     ON p.postId = c.postId 
ORDER BY p.postDate DESC, c.commentDate DESC;

我不是100％肯定会解决您的订购逻辑，但这至少应该返回更合理的结果集。就是说，我想按帖子和日期保持帖子的顺序，所以我将使用：

ORDER BY p.postDate DESC, p.postid, c.commentDate DESC;

这将使对具有相同postDate的帖子发表评论。

如何订购LEFT JOIN的结果

2 个答案:

先行

具有嵌套查询和顺序的UNION