我有两个表，如下所示：
表1（t1）
______________________________________________________________________________________________
RequestId | Raised_By | CommentDate | Comment | AttachmentName | Attachment | AttachmentSize |
----------+-----------+-------------+---------+----------------+------------+----------------+
表2（t2）
______________________________________________
RequestId | CommentDate | Comment | Raised_By |
----------+-------------+---------+-----------+
注意：
CommentDate是timestamp我想选择唯一的一条记录，即从表
t1或t2中注释，它们的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="."\"".$_POST['Raised_By']."\" )
ORDER BY CommentDate DESC
LIMIT 1
但是我说
unknown column CommentDate时出错。 最佳答案
如果尚未选择，可以ORDER BY吗？
我们需要了解查询执行的顺序。除非您选择字段'CommentDate'，否则您无法按顺序排列。
尝试选择'CommentDate'和'Comment'并检查
(SELECT Comment, CommentDate from t1 WHERE RequestId = "."\"".$_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