Question

以下查询选择所有帖子和每个帖子的所有者，属于每个帖子的所有评论以及每条评论的所有者。

我每篇文章只需检索5条评论。我重写了查询，但是我得到一个错误：“每个派生表必须拥有它自己的别名”。

SELECT posts.id AS postId, posts.body, users.id AS userId, users.displayname, comments.id AS commentId, comments.text, commenters.id, commenters.displayname
FROM posts
JOIN users ON posts.owneruserid = users.id
LEFT JOIN comments ON posts.id = comments.postid
    JOIN users AS commenters ON comments.userId = commenters.id
ORDER BY posts.createdAt

新查询：

SELECT posts.id AS postId, posts.body, users.id AS userId, users.displayname
FROM posts
JOIN users ON posts.owneruserid = users.id
LEFT JOIN (
    SELECT comments.id AS commentId, comments.text AS commentText, commenters.id AS commenterId, commenters.displayname AS commenterDisplayName
    FROM comments
    JOIN users AS commenters ON comments.userid = commenters.id
    LIMIT 0,5
        ) AS comments ON comments.postid = posts.id
ORDER BY posts.createdAt

更新查询现在有效，但不会产生所需的输出。我想输出10个帖子，每个帖子有5条评论。此限制条款仅适用于遇到的第一篇帖子的评论。

Answer 1

您需要为派生表提供别名：

SELECT posts.id AS postId, posts.body, users.id AS userId, users.displayname
FROM posts
JOIN users ON posts.owneruserid = users.id
LEFT JOIN (
        SELECT comments.id AS commentId, comments.text AS commentText, commenters.id AS commenterId, commenters.displayname AS commenterDisplayName
        FROM comments
        JOIN users AS commenters ON comments.userid = commenters.id
        LIMIT 0,5
    ) AS derived_table_alias
ORDER BY posts.createdAt

Answer 2

由于您正在使用子查询（这是“派生表”的含义），因此它必须具有别名。因此，您需要做的就是：

SELECT posts.id AS postId, posts.body, users.id AS userId, users.displayname
FROM posts
JOIN users ON posts.owneruserid = users.id
LEFT JOIN (
    SELECT comments.id AS commentId, comments.text AS commentText, commenters.id AS commenterId, commenters.displayname AS commenterDisplayName
    FROM comments
    JOIN users AS commenters ON comments.userid = commenters.id
    LIMIT 0,5
) as some_alias --This is what's triggering the error
ORDER BY posts.createdAt

即使您没有从子查询中进行选择，只是将其用作过滤器，也必须使用别名。

Answer 3

从编辑和评论反馈，这里是我认为你正在寻找的查询...最内部的预先查询获取帖子，谁发起了帖子，评论和谁发表评论。此内部查询也预先排序，每个postID的最顶层注释最多。使用结果，我加入sql变量（@variables）以在每次新注释时增加@varRow，并在每次帖子ID更改时重置为1（因此内部PreQuery命令通过帖子ID FIRST ）。最后，使用HAVING子句使注释的@varRow计数＆lt; 6将获得每个帖子的第5个。

如果要限制要检索的帖子，我会在生成“PreQuery”的INNER中应用WHERE子句（如可用的日期/时间）。

select straight_join
      PreQuery.*,
      @varRow := if( @LastPost = PreQuery.PostID, @varRow +1, 1 ) CommentRow,
      @LastPost := PreQuery.PostID PostID2
   from
      ( select
              posts.id PostID,
              posts.body,
              posts.CreatedAt,
              u1.id UserID,
              u1.DisplayName NameOfPoster,
              c.id,
              c.userid CommentUserID,
              c.text CommentText,
              u2.DisplayName CommentUserName
           from
              posts
                 join users u1
                    on posts.ownerUserID = u1.id

                 LEFT JOIN comments c
                    on posts.id = c.PostID

                    join users u2
                       on c.userid = u2.id 
            where
                  posts.id = TheOneParentIDYouWant
               OR posts.parentid = TheOneParentIDYouWant
            order by
               posts.ID,
               c.id desc ) PreQuery,

      (select @varRow := 0, @LastPost = 0 ) SQLVars

   having
      CommentRow < 6   

   order by
      PreQuery.postid,
      CommentRow

---编辑---每条评论我认为你的意思是与评论相关联的“父帖”是因为他们直接拥有帖子ID。由于最里面的查询会全面地连接所有元素/表格，所以所有元素/表格都会随之而来...

Post -> User (to get posting user name )
Post -> Comment (on Common Post ID -- left joined)
        Comment -> User ( to get commenting user name)

一旦完成全部操作并按照常用的帖子ID排序，并将最新的评论排序到顶部，然后我将@vars应用于所有返回的行。 HAVING子句将删除任何注释，其中的序列超出了您所寻找的5个。

Answer 4

错误

在子查询后添加别名。
示例：SELECT * FROM foo JOIN (select * from bar) AS <alias_here>
确保posts表中有一个字段，并将其称为createdAt。我不确定MySQL是否区分大小写，但您发布的错误显示createdat（小写'A'）

您有两个LEFT JOIN，但只有一个ON语句。连接不是没有钩子加入它的任何东西。例如：

SELECT *   
FROM foo JOIN bar ON (foo.id=bar.id) 
LEFT JOIN (select * from foobar) AS baz **ON foo.id=baz.id**

为了加入某个字段，该字段需要出现在连接所涉及的表中。因此，在上面的示例中，如果您将foo.id与baz.id匹配，则需要在子查询（baz）中返回id。

在mysql中使用左连接的限制

4 个答案:

错误