Query in Order By clause used with UNION

Question

When tryna execute the below qurey I get
ORA-01784 - ORDER BY item must be the number of a SELECT-list expression
Please help me resolve this asap.
SELECT cm_workqueue.WQUEUEID_REF,cm_workqueue.WQUEUEID_CODE,cm_workqueue.mstr_queue_encode,cm_workqueue.desc_text
FROM cm_workqueue,
cm_wkqueue_role,
atrt_role
WHERE cm_workqueue.MSTR_QUEUE_ENCODE = 10000000
AND cm_workqueue.WQUEUEID_REF = cm_wkqueue_role.WQUEUEID_REF
AND atrt_role.ROLEID_REF = cm_wkqueue_role.roleid_ref
AND cm_wkqueue_role.REFERRAL_YORN_ENCODE = 10000000
AND cm_workqueue.STATUS_REF = 10000000
AND cm_wkqueue_role.STATUS_REF = 10000000
UNION
SELECT cm_workqueue.WQUEUEID_REF,cm_workqueue.WQUEUEID_CODE,cm_workqueue.mstr_queue_encode,cm_workqueue.desc_text
FROM cm_workqueue,
cm_wkqueue_role,
atrt_role
WHERE cm_workqueue.mstr_queue_encode = 10000001
AND cm_workqueue.status_ref = 10000000
AND cm_wkqueue_role.wqurolid_ref = 1
AND EXISTS (
SELECT 1
FROM cm_filterset f,
cm_wkqueue_role cmr
WHERE f.wqueueid_ref = cm_workqueue.wqueueid_ref
AND f.status_ref = 10000000
AND cmr.roleid_ref = atrt_role.roleid_ref
AND cmr.wqueueid_ref = f.mstrquid_ref
AND cmr.referral_yorn_encode = 10000000
AND cmr.status_ref = 10000000)
ORDER BY CASE
WHEN cm_workqueue.wqueueid_ref < 999 THEN 1
ELSE cm_workqueue.mstr_queue_encode
END,
LOWER (cm_workqueue.desc_text)

Answer 1

To be correct, the column names in the order by must be the name of the column alias or the number of the selected column. If you don't give a column alias, the column name of the first select is taken.
SQL> create table t1 (a varchar2(1));
Table created.
SQL> create table t2 (aa varchar2(1));
Table created.
SQL> select a aa from t1
  2  union
  3  select aa from t2
  4  order by aa;
no rows selectedDid you see how we nicely could order by the column name of the second select? NOT! It's because of the column alias. Look at the next select
SQL> select a aa from t1
  2  union
  3  select aa from t2
  4  order by a;
order by a
ERROR at line 4:
ORA-00904: "A": invalid identifierDid you see, that the column name of the first select is not valid as there is a column alias?
That's why the best is, to use result set numbers when ordering in unions
SQL> select a aa from t1
  2  union
  3  select aa from t2
  4  order by 1;
no rows selected
SQL> Edited by: Leo Mannhart on Mar 6, 2009 10:37 AM
And that's why the columns in the order by MUST be in the select list. Another error in the original post