Question

查询非常简单，如下所示：

SELECT postId, postContent, postDate FROM posts UNION 
ALL SELECT commentId, commentContent, commentDate FROM comments 
ORDER BY postDate DESC, commentDate DESC

相反，按顺序排列结果会得到mysqli警告mysqli_fetch_assoc() expects parameter 1 to be mysqli_result, bool given

请帮助。

php代码如下

$query = mysqli_query($conn, $query);
while($row = mysqli_fetch_assoc($query)) {
echo $row['postId'];
echo $row['postContent'];
echo $row['postDate'];
}

postId    postContent     postDate
-------------------------------------
1    |   sample post1    | 03-02-2019
-------------------------------------
2    |  sample post 2    | 04-02-2019

这是帖子这是评论

commentId    commentContent     commentDate
--------------------------------------------
1        |   sample comment1    | 03-02-2019
--------------------------------------------
2        |  sample comment2     | 04-02-2019

Answer 1

考虑使用内部查询，前提是所有三列的数据类型相同，和
从, commentDate DESC子句中删除ORDER BY，因为commentDate也被认为是postDate。

SELECT q.*
  FROM
  ( 
   SELECT postId, postContent, postDate FROM posts
   UNION ALL
   SELECT commentId, commentContent, commentDate FROM comments 
   ) q
 ORDER BY postDate DESC

Answer 2

这是您的查询：

SELECT postId, postContent, postDate
FROM posts
UNION ALL
SELECT commentId, commentContent, commentDate
FROM comments 
ORDER BY postDate DESC, commentDate DESC

没有ORDER BY，结果集将包含三列，分别为名称postId，postContent和postDate。这些可以在ORDER BY中使用。实际上，commentDate将在结果集中重命名为postdate。

因此，此ORDER BY将起作用：

ORDER BY postDate DESC

不过，我怀疑那是您真正想要的。如果我推测一下，我会发现以下有用：

SELECT p.postId, p.postDate, p.postContent, NULL as commentDate, NULL as commentContent
FROM posts p
UNION ALL
SELECT p.postId, p.postDate, c.commentId, c.commentContent, c.commentDate
FROM comments c JOIN
     posts p
     ON c.postId = p.postId
ORDER BY postDate DESC, postId DESC, commentDate DESC

通过每个选择中的两个不同列对两个选择进行联合的顺序

2 个答案: