Question

我有两张表如下：

表1（t1）

______________________________________________________________________________________________
RequestId | Raised_By | CommentDate | Comment | AttachmentName | Attachment | AttachmentSize |
----------+-----------+-------------+---------+----------------+------------+----------------+

表2（t2）

______________________________________________
RequestId | CommentDate | Comment | Raised_By |
----------+-------------+---------+-----------+

注意：CommentDate是timestamp

我想选择唯一一条记录，即表格t1或t2的评论，其中RequestId=RequestId和CommentDate是最新的。

我的查询如下：

(SELECT Comment from t1 WHERE RequestId = "."\"".$_POST['RequestId']."\" and Raised_By="."\"".$_POST['Raised_By']."\" and CommentDate="."\"".$_POST['CommentDate']."\")
UNION 
(SELECT Comment from t2 WHERE RequestId = "."\"".$_POST['RequestId']."\" and Raised_By="."\"".$_POST['Raised_By']."\" )
ORDER BY CommentDate DESC
LIMIT 1

但我收到错误unknown column CommentDate。

Answer 1

如果你没有选择那个，你可以ORDER BY吗？我们需要了解查询执行的顺序。除非您选择字段'CommentDate'，否则无法按顺序排列。

尝试选择'CommentDate'和'Comment'，然后选中

(SELECT Comment, CommentDate from t1 WHERE RequestId = "."\"".$_POST['RequestId']."\" and Raised_By="."\"".$_POST['Raised_By']."\" and CommentDate="."\"".$_POST['CommentDate']."\")
UNION 
(SELECT Comment, CommentDate from t2 WHERE RequestId = "."\"".$_POST['RequestId']."\" and Raised_By="."\"".$_POST['Raised_By']."\" )
ORDER BY CommentDate DESC
LIMIT 1

Answer 2

为什么在UNION两个表都可以使用JOIN？

SELECT t1.Comment,
    CASE WHEN t1.CommetDate > t2.CommentDate
    THEN t1.commentDate
    ELSE t2.commentDate
    END AS commentDate
  FROM t1 INNER JOIN t2 ON t1.RequestId = t2.RequestId
  ORDER BY commentDate DESC
  LIMIT 1

根据两个不同表中的字段对行进行排序

2 个答案: