根據兩個不同表中的字段對行進行排序

Question

我有兩個表，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是一個timestamp

我想選擇唯一的一條記錄，即表t1或t2注釋，其RequestId=RequestId和CommentDate是最新的。

我的查詢如下：

(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

但是我收到一個錯誤消息，說unknown column CommentDate 。

Answer 1

如果您沒有選擇，可以ORDER BY嗎？ 我們需要了解查詢執行的順序。 除非您選擇'CommentDate'字段'CommentDate'無法按順序排列。

嘗試選擇'CommentDate'和'Comment'並檢查

(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

Answer 2

可以同時JOIN兩個表時為何使用UNION ？

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