sql syntax group by - sql

Question

Struggling with this as i'm not good with sql and designer wont work with the OVER use. Basically this is getting a list of topics if the user is following an associated tag.
I need to group by T.TopicId to stop duplicates. If a user is selecting more than one tag associated with a topic it will list the topic twice (once for each tag)
When I add a group by in sql I get multiple errors and i've tried rearranging things and cant get it to work, As said i'm useless with sql statements
#id int = 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, T.UserName, M.MessageCreationDate, M.UserId AS MessageUserId, MU.UserName AS MessageUserName, U.UserGroupId,
U.UserPhoto, T.UserFullName
FROM Tags INNER JOIN
TopicsComplete AS T ON T.TopicId = Tags.TopicId LEFT OUTER JOIN
Messages AS M ON M.TopicId = T.TopicId AND M.MessageId = T.LastMessageId AND M.Active = 1 LEFT OUTER JOIN
Users AS MU ON MU.UserId = M.UserId LEFT OUTER JOIN
Users AS U ON U.UserId = T.UserId LEFT OUTER JOIN
tagfollows AS TF ON #id = TF.userid
WHERE (Tags.Tag = TF.tag)
)T
If anyone could help it would be much appreciated, thanks! :)

Answer 1

I think you only need to convert the join to tagfollows into an EXISTS subquery (and remove the redundant nesting):
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,
T.UserName, M.MessageCreationDate,
M.UserId AS MessageUserId,
MU.UserName AS MessageUserName,
U.UserGroupId, U.UserPhoto, T.UserFullName
FROM
TopicsComplete AS T
LEFT OUTER JOIN
Messages AS M ON M.TopicId = T.TopicId
AND M.MessageId = T.LastMessageId
AND M.Active = 1
LEFT OUTER JOIN
Users AS MU ON MU.UserId = M.UserId
LEFT OUTER JOIN
Users AS U ON U.UserId = T.UserId
WHERE EXISTS
( SELECT *
FROM Tags
INNER JOIN tagfollows AS TF
ON Tags.Tag = TF.tag
WHERE T.TopicId = Tags.TopicId
AND #id = TF.userid
) ;

Answer 2

You say you want to show posts with tags in the set that the user is following, but you don't want the post to show up multiple times when it has multiple matching tags. That's a perfect use for an EXISTS subquery. Here's an example from that MSDN page.
SELECT a.FirstName, a.LastName
FROM Person.Person AS a
WHERE EXISTS
(SELECT *
FROM HumanResources.Employee AS b
WHERE a.BusinessEntityID = b.BusinessEntityID
AND a.LastName = 'Johnson');
You're really interested in the person table (like your posts table), but you want to show records that have at least one matching record in employee (like your tags table).