FULL OUTER JOIN with two subselects gives "ORA-00918: column ambiguously defined"

Question

both Subselects runs fine standalone.

But I wish to join them togehter, which always gives "ORA-00918: column ambiguously defined". I don't see why?

Both subelects uses other names.

SELECT *
  FROM    (SELECT aods.user_id,
                  aods.firstname,
                  aods.lastname,
                  Aods.DEPARTMENT,
                  cods.jc_name
             FROM    personas aods
                  LEFT JOIN
                     user_jc cods
                  ON aods.user_id = cods.user_id
            WHERE cods.jc_name LIKE '%ADM%') ods
       FULL OUTER JOIN
          (SELECT abl.user_id,
                  abl.firstname,
                  abl.lastname,
                  Abl.DEPARTMENT,
                  bbl.ug_name
             FROM    personas abl
                  LEFT JOIN
                     ru_ug bbl
                  ON abl.user_id = bbl.user_id
            WHERE Bbl.UG_NAME LIKE '%ADM%'
                  AND bbl.rss_name = 'TIR') bl
       ON ods.user_id = bl.user_id

Answer 1

Because you do a select * at the top.

user_id
firstname
lastname
DEPARTMENT

are present in both subqueries, and with select *it would get two columns with the same name.

If you want to merge the two subqueries (such that the resulting set of columns would be user_id, firstname, lastname, DEPARTMENT, jc_name, ug_name) you should do a natural join:

SELECT *
  FROM (SELECT aods.user_id, aods.firstname, aods.lastname, aods.department,
               cods.jc_name
          FROM personas aods LEFT JOIN user_jc cods
               ON aods.user_id = cods.user_id
         WHERE cods.jc_name LIKE '%ADM%') ods
       NATURAL JOIN
       (SELECT abl.user_id, abl.firstname, abl.lastname, abl.department,
               bbl.ug_name
          FROM personas abl LEFT JOIN ru_ug bbl ON abl.user_id = bbl.user_id
         WHERE bbl.ug_name LIKE '%ADM%' AND bbl.rss_name = 'TIR') bl

Instead if you want all the different columns, you should explicitly list them:

 SELECT ods.user_id, ods.firstname, ods.lastname, ods.department, ods.jc_name,
        bl.user_id, bl.firstname, bl.lastname, bl.department, bl.ug_name
   FROM (SELECT aods.user_id, aods.firstname, aods.lastname, aods.department,
                cods.jc_name
           FROM personas aods LEFT JOIN user_jc cods
                ON aods.user_id = cods.user_id
          WHERE cods.jc_name LIKE '%ADM%') ods
        FULL OUTER JOIN
        (SELECT abl.user_id, abl.firstname, abl.lastname, abl.department,
                bbl.ug_name
           FROM personas abl LEFT JOIN ru_ug bbl ON abl.user_id = bbl.user_id
          WHERE bbl.ug_name LIKE '%ADM%' AND bbl.rss_name = 'TIR') bl
        ON ods.user_id = bl.user_id

In your comment you say that you want to use nvl() on each column with an outer join. For me it works fine:

SELECT NVL (ods.user_id, bl.user_id), NVL (ods.firstname, bl.firstname),
       NVL (ods.lastname, bl.lastname), NVL (ods.department, bl.department),
       ods.jc_name, bl.ug_name
  FROM (SELECT '1' user_id, 'ods2' firstname, NULL lastname,
               'ods3' department, 'ods4' jc_name
          FROM DUAL) ods
       FULL OUTER JOIN
       (SELECT '1' user_id, NULL firstname, 'bl2' lastname, 'bl3' department,
               'bl4' ug_name
          FROM DUAL) bl ON ods.user_id = bl.user_id

are you making some additional stuff, like order by?