Sql Joins in the same table

Question

Hi, I have a table (DFO_TRACE) that has two columns CURRENT_FID and SOURCE_FID and I want to find out what are the values that are present in the CURRENT_FID that are not present in the SOURCE_FID for example:
SOURCE_FID | CURRENT_FID
1 | 2
2 | 3
2 | 4
4 | 5
So in the example above I would like to do a select that would return CURRENT_FID = 3 and 5.
I've tried to write the following code to do this, but when I execute it, it doesn't return anything to me:
select A.CURRENT_FID from DFO_TRACE A
left outer join DFO_TRACE B
on A.CURRENT_FID = B.SOURCE_FID
where A.ID_GRUPO_TRACE = 177 and B.ID_GRUPO_TRACE = 177 and B.SOURCE_FID is null;Can anyone help me?
PS: I am using Oracle 10g.
Thanks,
Komyg

Answer 1

The reason you posted query does not work is that the predicate B.ID_GRUPO_TRACE = 177 in the where clause removes all od the "made_up" records from the b version of the table since their id_grupo_trace would be null. You would need to modify your query to something more like:
SELECT a.current_fid
FROM dfo_trace a
   LEFT JOIN dfo_trace b
      ON a.current_fid = b.source_fid and
         a.id_grupo_trace = b.id_grupo_trace
WHERE a.id_grupo_trace = 177 and
      b.source_fid IS NULL;John