Scalar Select stmt in Case causing problems

Question

Hello,
here is some background followed by my question
I have two tables, headerx and detailx which are joined
Detailx has a sequence number column
I would like to query detailx such that if for any header there are more than one details, it should return the highest sequence with state = passed.
If there is only one detail, then it should return that detail row.
So, I came up with this SQL:
select DTL.ID, DTL.TSTAMP
from HEADERX hdr inner join
     DETAILX dtl on DTL.HDR_FK = hdr.id
where dtl.seq =
     (select case max(seq) when 1 then 1
                                   else (select max(seq) from DETAILX dtl3 where dtl3.hdr_fk = hdr.id and dtl3.state='PASSED')
                                   end
       from DETAILX dtl2
       where dtl2.hdr_fk = hdr.id)
; The above threw a ORA 937 - Not a single-group group function
If I remove the embedded select inside the case and replace it with an arbitrary value, there is no error and some rows are returned
select DTL.ID, DTL.TSTAMP
from HEADERX hdr inner join
     DETAILX dtl on DTL.HDR_FK = hdr.id
where dtl.seq =
     (select case max(seq) when 1 then 1 else 2 end from DETAILX dtl2 where dtl2.hdr_fk = hdr.id)
; Not really understanding why, I added a group by which appears to work
select DTL.ID, DTL.TSTAMP
from HEADERX hdr inner join
     DETAILX dtl on DTL.HDR_FK = hdr.id
where dtl.seq =
     (select case max(seq) when 1 then 1
                                    else (select max(seq) from DETAILX dtl3 where dtl3.hdr_fk = hdr.id and dtl3.state='PASSED')
                                    end
      from DETAILX dtl2
      where dtl2.hdr_fk = hdr.id
      group by 1)
; Question: Why is the group by required? Clearly there is only one scalar value that will be returned
If anyone cares to suggest, is there a more elegant solution?

Answer 1

Instead of joining the same table too many times,
You can do this,
Not tested,
SELECT DTL.ID, DTL.TSTAMP
  FROM    HEADERX hdr
       INNER JOIN
          (SELECT dtl.*, CASE WHEN MAX (seq) OVER (PARTITION BY ID) = 1
                              THEN 1
                              ELSE MAX (DECODE (state, 'PASSED', seq)) OVER (PARTITION BY ID)
                          END mx
             FROM DETAILX dtl) dtl
       ON (DTL.HDR_FK = hdr.ID
       AND hdr.seq = dtl.mx)
G.