如果其他列爲空，則按一列排序

Question

我有兩個表：一個主題表和一個連接的評論表，都帶有時間戳列。我想訂購主題列表，以便最新評論的主題位於頂部。但是，如果主題沒有評論，我希望它按創建日期排序。 換句話說我想，如果他們有沒有意見訂購它，這樣的主題是由他們創造僅日期排列 - 基本上就像任何論壇。例如，如果在主題B（非空）之後創建了主題A（空），但主題B具有最近的回覆，那麼順序應該是B，A.我不希望頂部的所有空主題如果如果它們是新的，它們就會老舊或者處於底部。

我試過IF ISNULL聲明，但它適用於整列，而不是每個單排，所以我結束與空線程要麼停留在飼料的頂部或底部。

我猜我不得不構建，只有從每個主題的最新評論一個虛擬列...？

以下是聲明全文：

SELECT 
    $showbody, 
    Topics.Title, 
    Topics.id AS tID, 
    Topics.Timestamp, 
    Topics.MemberID, 
    Users.id, 
    Users.FirstName, 
    Users.LastName, 
    GROUP_CONCAT(DISTINCT Tags.Keywords SEPARATOR ', ') AS Tags, 
    COUNT(Comments.id) AS NumberOfComments, 
    (
    SELECT COUNT(Comments.id) 
    FROM Comments 
     LEFT JOIN Views ON Comments.TopicID = Views.TopicID 
    WHERE Comments.Timestamp > Views.Visited 
) AS NewComments 
FROM Topics 
    LEFT JOIN Users ON Topics.MemberID = Users.ID 
    LEFT JOIN Comments ON Topics.id = Comments.TopicID 
    LEFT JOIN Tags ON Topics.id = Tags.TopicID 
WHERE Topics.id NOT IN (
    SELECT Tags.TopicID 
    FROM Tags 
    WHERE Keywords IN (
    SELECT Tag 
    FROM Filters 
    WHERE MemberID = '$_SESSION[SESS_MEMBER_ID]' 
) 
    GROUP BY Tags.TopicID 
) 
GROUP BY Topics.id 
ORDER BY Comments.Timestamp, Topics.Timestamp DESC LIMIT $plim 

任何幫助，將不勝感激

Answer 1

溶液1（演示）：

-- Test initialization 
CREATE TABLE Question 
(
    QuestionID INT AUTO_INCREMENT PRIMARY KEY 
    ,Content NVARCHAR(100) NOT NULL 
    ,CreateDate DATETIME NOT NULL -- DEFAULT NOW() 
); 

CREATE TABLE QuestionComment 
(
    QuestionCommentID INT AUTO_INCREMENT PRIMARY KEY 
    ,QuestionID INT NOT NULL REFERENCES Question(QuestionID) 
    ,Content NVARCHAR(100) NOT NULL 
    ,CreateDate DATETIME NOT NULL -- DEFAULT NOW() 
); 

INSERT Question(Content, CreateDate) 
SELECT 'Question 1','2011-01-01 01:00:00' 
UNION ALL 
SELECT 'Question 2','2011-01-01 02:00:00' 
UNION ALL 
SELECT 'Question 3','2011-01-01 03:00:00'; 

INSERT QuestionComment(QuestionID, Content, CreateDate) 
SELECT 1,'Comment 1.1','2011-01-01 01:30:00' 
UNION ALL 
SELECT 2,'Comment 2.1','2011-01-01 02:30:00' 
UNION ALL 
SELECT 1,'Comment 1.2','2011-01-01 02:40:00' 
UNION ALL 
SELECT 2,'Comment 2.2','2011-01-01 02:30:00' 
UNION ALL 
SELECT 1,'Comment 1.3','2011-01-01 03:30:00'; 
-- End of Test initialization 

-- Solution 
SELECT * 
FROM Question q 
LEFT JOIN 
(
    SELECT qc.QuestionID, MAX(qc.CreateDate) LastCreateDate 
    FROM QuestionComment qc 
    GROUP BY qc.QuestionID 
) qc ON q.QuestionID=qc.QuestionID 
ORDER BY IFNULL(qc.LastCreateDate, q.CreateDate) ASC; 
-- End of Solution 

-- By, by 
DROP TABLE QuestionComment; 
DROP TABLE Question; 

結果：

2 Question 2 2011-01-01 02:00:00 2 2011-01-01 02:30:00 
3 Question 3 2011-01-01 03:00:00  
1 Question 1 2011-01-01 01:00:00 1 2011-01-01 03:30:00 

解決方案2：

SELECT * 
FROM Topics t 
LEFT JOIN 
(
    SELECT c.TopicID, MAX(Timestamp) LastTimestamp 
    FROM Comments c 
    GROUP BY c.TopicID 
) c ON t.id = c.TopicID 
ORDER BY IFNULL(c.LastTimestamp, t.Timestamp) ASC 

Answer 2

使用LEFT JOIN的意見，對課題小組，使用max拿到最後一個註釋的時候，並使用coalesce（或isnull）對註釋日期進行排序，如果它存在的話，否則就是創建日期。

實施例：

select t.Title, t.CreatedDate, max(c.CreatedDate) as LastComment 
from Topics t 
left join Comment c on c.TopicId = t.TopicId 
group by t.Title, t.CreatedDate 
order by coalesce(max(c.CreatedDate), t.CreatedDate) desc