Using ROW_NUMBER() to remove duplicates in SQL server - sql-server

Question

My current query returns too many lines per Subject_ID, so I want to use ROW_NUMBER() to limit the resulting set to 1 line per Subject_ID. I've added this line to my SELECT statement:
, ROW_NUMBER() over(partition by CS.Subject_ID order by CS.Subject_ID) rn
But when I try to put WHERE rn = 1 anywhere in the FROM statement, I get the error:
Incorrect syntax near the keyword 'WHERE'
And when I try to change it to AND rn = 1 (and add it on to another AND/OR line) I get the error:
Invalid column name 'rn'
So my first question is: When I add a field to my SELECT statement using that ROW_NUMBER() line, what table does this column belong to? Do I need to append it to something like Table.rn? My second question is where should I put this rn = 1 line and how should I write it in?
Full query:
SELECT
Groups.Group_Name
, CT.Created
, CT.Subject_Id
INTO #temp
FROM SubjectZ_Task CT
INNER JOIN
SubjectZ_Task_Users On CT.SubjectZ_Task_Id = SubjectZ_Task_Users.SubjectZ_Task_Id
INNER JOIN
Groups ON Groups.Group_ID = SubjectZ_Task_Users.Group_Reference
WHERE Group_Name LIKE 'Team 1'
AND CT.Created >= '1/1/2019' AND CT.Created < DATEADD(Day,1,'12/31/2019')
GROUP BY Groups.group_name, CT.Created, CT.Subject_ID
SELECT
CT.Group_Name
, CT.Created
, CS.Topic_Start_Date
, CS.Subject_ID
, P.FirstName
, P.LastName
, CS.Subject_Match_ID
, SubjectX.Firstname AS SubjectX_firstname
, CS.SubjectY
, AEC.AEC AS Max_AEC
, SubjectX.Email_id As SubjectX_Email
, Phone.Phone
, ROW_NUMBER() over(partition by CS.Subject_ID order by CS.Subject_ID) rn
FROM #temp CT
LEFT JOIN QE_Topic_Summary CS ON CS.Subject_ID = CT.Subject_Id
AND (Topic_Status LIKE 'In Progress'
OR Topic_Status LIKE 'Pending')
AND CS.Topic_Start_Date >= DATEADD(Day,-60,CT.Created) AND CS.Topic_Start_Date <= DATEADD(Day,60,CT.Created)
INNER JOIN Subjects P ON P.Subject_ID = CS.Subject_ID
LEFT JOIN Subjects SubjectX ON SubjectX.Subject_ID = CS.SubjectX_ID
LEFT JOIN QE_TB_MAX_AEC AEC ON AEC.Subject_ID = CS.Subject_ID
INNER JOIN Subject_Identifiers PI ON PI.Subject_ID = P.Subject_ID
LEFT JOIN Subject_Identifiers PIP ON PIP.Subject_ID = SubjectX.Subject_ID
LEFT JOIN Subject_Phone Phone On Phone.Subject_ID = P.Subject_ID WHERE Phone.Voice = 1
drop table #temp

Answer 1

I don't see a reference to rn in your WHERE clause, but my guess is that you need to wrap it in another query like so:
SELECT *
FROM(
SELECT
CT.Group_Name
, CT.Created
, CS.Topic_Start_Date
, CS.Subject_ID
, P.FirstName
, P.LastName
, CS.Subject_Match_ID
, SubjectX.Firstname AS SubjectX_firstname
, CS.SubjectY
, AEC.AEC AS Max_AEC
, SubjectX.Email_id As SubjectX_Email
, Phone.Phone
, ROW_NUMBER() over(partition by CS.Subject_ID order by CS.Subject_ID) rn
FROM #temp CT
LEFT JOIN QE_Topic_Summary CS ON CS.Subject_ID = CT.Subject_Id
AND (Topic_Status LIKE 'In Progress'
OR Topic_Status LIKE 'Pending')
AND CS.Topic_Start_Date >= DATEADD(Day,-60,CT.Created) AND CS.Topic_Start_Date <= DATEADD(Day,60,CT.Created)
INNER JOIN Subjects P ON P.Subject_ID = CS.Subject_ID
LEFT JOIN Subjects SubjectX ON SubjectX.Subject_ID = CS.SubjectX_ID
LEFT JOIN QE_TB_MAX_AEC AEC ON AEC.Subject_ID = CS.Subject_ID
INNER JOIN Subject_Identifiers PI ON PI.Subject_ID = P.Subject_ID
LEFT JOIN Subject_Identifiers PIP ON PIP.Subject_ID = SubjectX.Subject_ID
LEFT JOIN Subject_Phone Phone On Phone.Subject_ID = P.Subject_ID
WHERE Phone.Voice = 1
)t
WHERE t.rn = 1