SQL：論壇整理行爲（CI中）

Question

這裏的情況：

在我的數據庫我有螺紋表，並與回覆的表格。兩者都有一個時間戳字段。

現在我正在開發一個論壇，並希望通過以下方式訂購主題：

• 如果線程有回覆，則：ORDER BY tblReply.Timestamp DESC
• 否則，線程沒有回覆：ORDER BY tblThread.Timestamp DESC

我不知道如何在一個語句中合併這2個。

我的查詢，因爲它現在是：

SELECT `PK_ThreadID`, `Title`, `tblUsers`.`Username`, `tblThread`.`Date`, count(tblReply.FK_ThreadID) AS number_replies FROM (`tblThread`) 
JOIN `tblUsers` ON `tblUsers`.`PK_UserID` = `tblThread`.`FK_UserID` 
LEFT JOIN `tblReply` ON `tblReply`.`FK_ThreadID` = `tblThread`.`PK_ThreadID` 
WHERE `isExpertQuestion` = 0 AND `isPublic` = 1 
GROUP BY `PK_ThreadID` 
ORDER BY max(tblReply.Date)` desc 

//Here it only orders by reply date, so threads with no replies appear at the bottom 

我要如何實現，我想在此查詢的順序？

Answer 1

喜歡這大概是：

SELECT `PK_ThreadID`, `Title`, 
`tblUsers`.`Username`, 
`tblThread`.`Date`, 
count(tblReply.FK_ThreadID) AS number_replies 
FROM (`tblThread`) 
JOIN `tblUsers` ON `tblUsers`.`PK_UserID` = `tblThread`.`FK_UserID` 
LEFT JOIN `tblReply` ON `tblReply`.`FK_ThreadID` = `tblThread`.`PK_ThreadID` 
WHERE `isExpertQuestion` = 0 AND `isPublic` = 1 
GROUP BY `PK_ThreadID` 
ORDER BY 
CASE WHEN COUNT(tblReply.FK_ThreadID) > 0 THEN tblReply.Timestamp 
    WHEN COUNT(tblReply.FK_ThreadID) = 0 OR tblReply.FK_ThreadID IS NULL 
THEN tblThread.Timestamp 
END DESC