GROUP BY in SQL Server in complex query - sql-server

Question

I need to group this by T.TopicID to only receive the last result.
Whatever I try I get errors like the other T. items rant included in group by or aggregate etc
ALTER PROCEDURE [dbo].[SPGetFollowingTopics]
#id int = null
,#UserGroupId int = null
,#lastvisit DateTime = null
AS
SELECT *
FROM
(SELECT
ROW_NUMBER() OVER (ORDER BY TopicOrder DESC,
(CASE
WHEN M.MessageCreationDate > T.TopicCreationDate
THEN M.MessageCreationDate
ELSE T.TopicCreationDate
END) DESC) AS RowNumber,
T.TopicId, T.TopicTitle, T.TopicShortName,
T.TopicDescription, T.TopicCreationDate, T.TopicViews,
T.TopicReplies, T.UserId, T.TopicTags, T.TopicIsClose,
T.TopicOrder, T.LastMessageId, U.UserName,
M.MessageCreationDate, T.ReadAccessGroupId,
T.PostAccessGroupId, TF.userid AS Expr1, U.UserGroupId,
U.UserPhoto, U.UserFullName, M.UserId AS MessageUserId,
MU.UserName AS MessageUserName
FROM
Topics AS T
LEFT OUTER JOIN
Messages AS M ON M.TopicId = T.TopicId AND M.Active = 1 AND M.MessageCreationDate < #lastvisit
INNER JOIN
topicfollows AS TF ON T.TopicId = TF.topicid
INNER JOIN
Users AS U ON U.UserId = T.UserId
LEFT JOIN
Users MU ON MU.UserId = M.UserId
WHERE
(TF.userid = #id)
) T

Answer 1

It isn't clear what the requirement is (in my view) but I think you are seeking:
"the latest message"
PER TOPIC
for a given user
In this situation ROW_NUMBER() is a good option but I believe you need to PARTITION the ROW_NUMBER as well as ordering it.
SELECT
*
FROM (
SELECT
ROW_NUMBER() OVER (PARTITION BY TF.userid, T.TopicId
ORDER BY
(CASE
WHEN M.MessageCreationDate > T.TopicCreationDate THEN M.MessageCreationDate
ELSE T.TopicCreationDate
END) DESC) AS ROWNUMBER
, T.TopicId, T.TopicTitle, T.TopicShortName, T.TopicDescription
, T.TopicCreationDate, T.TopicViews, T.TopicReplies, T.UserId
, T.TopicTags, T.TopicIsClose, T.TopicOrder, T.LastMessageId
, U.UserName, M.MessageCreationDate, T.ReadAccessGroupId
, T.PostAccessGroupId, TF.userid AS EXPR1
, U.UserGroupId, U.UserPhoto, U.UserFullName
, M.UserId AS MESSAGEUSERID, MU.UserName AS MESSAGEUSERNAME
FROM Topics AS T
LEFT OUTER JOIN Messages AS M ON M.TopicId = T.TopicId
AND M.Active = 1
AND M.MessageCreationDate < #lastvisit
INNER JOIN topicfollows AS TF ON T.TopicId = TF.topicid
INNER JOIN Users AS U ON U.UserId = T.UserId
LEFT JOIN Users MU ON MU.UserId = M.UserId
WHERE (TF.userid = #id)
) T
WHERE ROWNUMBER = 1

Answer 2

You could change your left join to any outer apply, and add TOP 1:
SELECT ...
FROM
Topics AS T
OUTER APPLY
( SELECT TOP 1 M.MessageCreationDate, M.UserId
FROM Messages AS M
WHERE M.TopicId = T.TopicId
AND M.Active = 1
AND M.MessageCreationDate < #lastvisit
ORDER BY M.MessageCreationDate DESC
) AS m
This allows you to use TOP 1 and still get one row per topicID
Alternatively you can use ROW_NUMBER() OVER(PARTITION BY m.TopicID ORDER BY M.MessageCreationDate DESC)
SELECT ...
FROM
Topics AS T
LEFT OUTER JOIN
( SELECT M.TopicId,
M.MessageCreationDate,
M.UserId,
RowNum = ROW_NUMBER() OVER(PARTITION BY m.TopicID ORDER BY M.MessageCreationDate DESC)
FROM Messages AS M
WHERE M.Active = 1
AND M.MessageCreationDate < #lastvisit
) AS m
ON M.TopicId = T.TopicId
AND m.RowNum = 1
I would test both methods and see which one works best for you.