Order by logic for a forum thread - sql

Question

I have three columns
ThreadID
DateTime
CommentID
ReplyCommentID
Query
WITH CTE AS ( SELECT CommentID ,
CommentUserName,
ReplyCommentID ,
CommentID AS ThreadID ,
CAST( CommentID AS VARCHAR( MAX ) ) AS PathStr,
HtmlComment ,
CommentPostDocumentID ,
CommentIsApproved,
CommentDate
FROM Blog_CommentDetails AS T WITH(NOLOCK)
WHERE ReplyCommentID IS NULL
UNION ALL
SELECT T.CommentID ,
T.CommentUserName,
T.ReplyCommentID ,
CTE.ThreadID ,
PathStr + '-'+ CAST( T.ReplyCommentID AS VARCHAR( MAX ) ) AS PathStr,
T.HtmlComment ,
t.CommentPostDocumentID ,
t.CommentIsApproved,
T.CommentDate
FROM Blog_CommentDetails AS T WITH(NOLOCK)
JOIN CTE
ON T.ReplyCommentID = CTE.CommentID
WHERE T.ReplyCommentID IS NOT NULL)
SELECT *
FROM CTE
WHERE CommentPostDocumentID = 15 AND CommentIsApproved=1
ORDER BY ThreadID, PathStr ,
CommentDate DESC;
I need to order by ThreadID ascending first
Then i need to order by CommentID ascending second
Then i need to order by date descending third
But one condision, when there is commenid and replycommendid matches for two rows, rows with commenid should be first.
How can i write an order by for this?
ORDER BY ThreadID,CommentID,DateTime desc,
IF(ReplyCommentID == CommentID)
then
rows with commentid should be first
Current result:
But the expected result is:

Answer 1

You can use a calculated value for ordering; However, as calculation requires attributes from separate rows, these rows have first to be combined.
Without demo data in textual form, which I could take over to my environment, it's a bit hard to test.
The following schema is very close to your requirements, if we consider a as representing CommentId and b standing for ReplyCommentID:
create table test (
a int,
b int
);
insert into test (a,b) values (1,null), (2,1), (3,2), (4,1);
select distinct test.a as a, test.b as b, case when (test.b=test2.a or test.b is null) then 0 else 1 end as priority
from test left join test test2 on test.b = test2.a and test2.b is null
order by priority,a,b
Note that the order compared to an order by a,b changes analogously to what you expect in your sample data.
Givent that, when applying to your query (after the WITH CTE...-part), it should look as follows. As mentioned above, I cannot test it, so please do not throw stones on me if it does not work immediately:
SELECT distinct cte.*, case when (cte.ReplyCommentID =cte2.CommentId or cte.ReplyCommentID is null) then 0 else 1 end as priority
FROM CTE left join CTE cte2 on cte.ReplyCommentID = cte2.CommentId and cte2.ReplyCommentID is null
WHERE CommentPostDocumentID = 15 AND CommentIsApproved=1
ORDER BY ThreadID, priority, CommentId, PathStr , CommentDate DESC;

Answer 2

The following extensive example will walk through a tree of comments based on CommentID and ReplyCommentID (which serves as the ParentID). Children of the same ReplyCommentID are ordered by CommentDate DESC:
DECLARE #Table TABLE (
CommentID INT,
ReplyCommentID INT,
CommentDate DATETIME
);
INSERT INTO #Table VALUES
(140,NULL, CAST('20170109' AS DATETIME))
,(141,NULL, CAST('20170110' AS DATETIME))
,(142,141, CAST('20170111' AS DATETIME))
,(143,141, CAST('20170112' AS DATETIME))
,(144,141, CAST('20170113' AS DATETIME))
,(145,144, CAST('20170114' AS DATETIME))
,(146,NULL, CAST('20170115' AS DATETIME));
WITH [Statistics] AS (
SELECT Records.CommentID, Records.ReplyCommentID
, COUNT(Children.CommentID) AS NrOfChildren
, ROW_NUMBER() OVER (PARTITION BY Records.ReplyCommentID ORDER BY Records.CommentDate DESC) AS NthChild
, COUNT(Records.CommentID) OVER (PARTITION BY Records.ReplyCommentID) AS NrOfSiblings
FROM #Table Records
LEFT JOIN #Table Children ON Records.CommentID = Children.ReplyCommentID
GROUP BY Records.CommentID, Records.ReplyCommentID, Records.CommentDate
)
, Tree AS (
SELECT *
, 1 AS [Order]
, 0 AS [Rerouting]
, CAST(-1 AS INT) AS ReroutedFromNthChild
FROM [Statistics] AS TreeNode
WHERE ReplyCommentID IS NULL AND NthChild = 1
UNION ALL
SELECT NextNode.*
, TreeNode.[Order] + 1 AS [Order]
, CASE
WHEN (TreeNode.NrOfChildren = 0 AND TreeNode.NthChild = TreeNode.NrOfSiblings AND TreeNode.ReplyCommentID = NextNode.CommentID)
OR (TreeNode.Rerouting = 1 AND TreeNode.ReroutedFromNthChild = TreeNode.NrOfChildren AND TreeNode.ReplyCommentID = NextNode.CommentID)
THEN 1
ELSE 0
END AS [Rerouting]
, CAST(TreeNode.NthChild AS INT) AS ReroutedFromNthChild
FROM Tree AS TreeNode
JOIN [Statistics] AS NextNode
--Has children, so select first child
ON (TreeNode.Rerouting = 0 AND TreeNode.NrOfChildren > 0 AND NextNode.NthChild = 1 AND TreeNode.CommentID = NextNode.ReplyCommentID)
--Has no children, so select next sibling
OR (TreeNode.Rerouting = 0 AND TreeNode.NrOfChildren = 0 AND TreeNode.NthChild + 1 = NextNode.NthChild AND (TreeNode.ReplyCommentID = NextNode.ReplyCommentID OR (TreeNode.ReplyCommentID IS NULL AND NextNode.ReplyCommentID IS NULL)))
--Has no children or following siblings, so retrace the step (reroute)
OR (TreeNode.Rerouting = 0 AND TreeNode.NrOfChildren = 0 AND TreeNode.NthChild = TreeNode.NrOfSiblings AND TreeNode.ReplyCommentID = NextNode.CommentID)
--Was rerouting but has children, so back on track and follow the next child
OR (TreeNode.Rerouting = 1 AND TreeNode.ReroutedFromNthChild < TreeNode.NrOfChildren AND TreeNode.ReroutedFromNthChild + 1 = NextNode.NthChild AND TreeNode.CommentID = NextNode.ReplyCommentID)
--Was rerouting and has no other children, so continue rerouting
OR (TreeNode.Rerouting = 1 AND TreeNode.ReroutedFromNthChild = TreeNode.NrOfChildren AND TreeNode.ReplyCommentID = NextNode.CommentID)
--Rerouted to the top without children left, jumping to the next sibling
OR (TreeNode.Rerouting = 1 AND (TreeNode.ReroutedFromNthChild = TreeNode.NrOfChildren OR TreeNode.NrOfChildren = 0) AND TreeNode.NthChild + 1 = NextNode.NthChild AND TreeNode.ReplyCommentID IS NULL AND NextNode.ReplyCommentID IS NULL)
)
select *
from Tree
where Rerouting = 0
ORDER BY [Order]