ORDER BY columns in a UNION

Question

Hi all, I have a pair of statements connected by a UNION, where I need to sort the final result according to columns not appearing in any column list:
SELECT REF(d) FROM DOCUMENT_TABLE d
WHERE d.header$.parentKey = ?
UNION
SELECT REF(d) FROM DOCUMENT_TABLE d, CROSS_TABLE k
WHERE k.doc = d.key$ AND k.folder = ?
ORDER BY d.header$.name
this fails saying that d.header$.name is an invalid identifier (ORA-904), like being out of scope.
Both union branches succeed when run alone (select + order by).
How can I sort by colums without having them to be returned ?
In all examples I found columns are also retrieved and referenced by either alias or by position.
Thanks.

Answer 1

Hi,
In a UNION query, you can only ORDER BY columns in the SELECT clause.
You can do the UNION in a sub-query, then do the ORDER BY in the main query, where you can ORDER BY anything:
WITH     union_results     AS
     SELECT  REF (d)     AS ref_d
     ,     name     
     FROM      DOCUMENT_TABLE     d
     WHERE      d.header$.parentKey = ?
UNION
        SELECT  REF (d)     AS ref_d
     ,     name
     FROM      DOCUMENT_TABLE     d
     ,     CROSS_TABLE     k
     WHERE     k.doc       = d.key$
     AND     k.folder  = ?
SELECT    REF_D
FROM       union_results
ORDER BY  name
;Your front end may be able to hide specified columns, so you could do this without a sub-query.
For example, in SQL*Plus you can say:
COLUMN  name    NOPRINT
     SELECT  REF (d)     AS ref_d
     ,     name     
     FROM      DOCUMENT_TABLE     d
     WHERE      d.header$.parentKey = ?
UNION
        SELECT  REF (d)     AS ref_d
     ,     name
     FROM      DOCUMENT_TABLE     d
     ,     CROSS_TABLE     k
     WHERE     k.doc       = d.key$
     AND     k.folder  = ?
ORDER BY  name
;The result set actually contains two columns, but SQL*Plus will only display one.