Tror ikke du kan bruke kolonne-aliaser i WHERE-clauser? (irriterende nok)

Hva om du prøver noe som ligner dette (mulig jeg er på feil jorde her, dog):

WITH C AS
(
select T.ThreadId AS TId, P.PostId AS PId,
ROW_NUMBER() over(partition by T.ThreadId order by P.PostId desc) AS RN
from Forum_Posts as P
join Forum_Threads as T
on T.ThreadId = P.ThreadId
order by P.PostId desc;
)
Select TId, PId, RN from C
ORDER BY PId DESC