Question

I have two tables: a Topics tables and a joined Comments tables, both with timestamp columns. I would like to order the topics list so that topics with the most recent comment are at the top. However, if a topic has no comments, I want it to be sorted by the date it was created.
In other words I’m trying to order it so that the topics are sorted by the date they were created only if they have no comments – basically like any forum. For example if topic A (empty) was created after topic B (not empty) but topic B has a reply that was most recent the order should be B, A. I don’t want all the empty topics at the top if they’re old or at the bottom if they’re new.

I tried the IF ISNULL statement but it applies to the entire column and not each individual row so I end up with the empty threads either stuck at the top or bottom of the feed.

I’m guessing I’d have to construct a virtual column with only the most recent comment from each topic…?

Here’s the full statement:

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

Any help would be greatly appreciated

Answer 1

Solution 1 (demo):

-- 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;

Results:

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

Solution 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