Using Both OrderBy And RowNum In Query

I tried to make my Query as simple as possible but also it contains the problem:

SELECT A.Id,A.Attachment,A.CreateDateTime,A.No
,rownum as RowNumber
FROM (
SELECT A.Id,A.Attachment,A.CreateDateTime,A.No FROM GOutgoingLetter A ,
(
SELECT B.Id, B.createdatetime as of0 FROM GOutgoingLetter B
WHERE
Exists
[code]...

when I use Both Order By And RowNum in my Query, Two columns of final Select Are Null: NO & Attachment. whereas this Columns aren't empty.when I comment each one of "ORDER BY B.of0" or ",rownum as RowNumber " everything is correct!!