Getting "column ambiguously defined" without any join in code - sql

Question

I get "column ambiguously defined" error while trying to execute following script:
INSERT INTO experience (id_, company, start_, end_, usr, position_)
WITH exp_ AS (
SELECT 0, 0, DATE'2013-04-30', NULL, 11, 'CEO' FROM DUAL UNION ALL
SELECT 1, 0, DATE'2011-12-11', DATE'2013-04-30', 9, 'Administrator' FROM DUAL UNION ALL
SELECT 2, 0, DATE'2013-04-30', DATE'2015-10-27', 6, 'Manager' FROM DUAL UNION ALL
SELECT 3, 1, DATE'2014-11-04', NULL, 10, 'CEO' FROM DUAL UNION ALL
SELECT 4, 1, DATE'2017-06-13', NULL, 13, 'CTO' FROM DUAL UNION ALL
SELECT 5, 1, DATE'2010-04-22', DATE'2018-10-22', 1, 'Manager' FROM DUAL UNION ALL
SELECT 6, 1, DATE'2019-08-26', NULL, 2, 'Trainer' FROM DUAL UNION ALL
SELECT 7, 1, DATE'2006-02-27', DATE'2016-11-30', 6, 'Video Creator' FROM DUAL UNION ALL
SELECT 8, 2, DATE'2012-04-30', DATE'2013-11-24', 5, 'CEO' FROM DUAL UNION ALL
SELECT 9, 2, DATE'2013-01-10', NULL, 0, 'CEO' FROM DUAL UNION ALL
SELECT 10, 2, DATE'2010-11-01', DATE'2017-01-01', 7, 'Video Creator' FROM DUAL UNION ALL
SELECT 11, 2, DATE'2010-10-03', DATE'2018-03-01', 8, 'Commentator' FROM DUAL
)
SELECT * FROM exp_;
I don't use any JOIN here, and the error description says, that it's caused by trying to join two tables with overlapping column's names.
So why I got it here and how to fix it?

Answer 1

This syntax would work only if you drop the CTE and directly insert the rows with UNION ALL:
INSERT INTO experience (id_, company, start_, end_, usr, position_)
SELECT 0, 0, DATE'2013-04-30', NULL, 11, 'CEO' FROM DUAL UNION ALL
SELECT 1, 0, DATE'2011-12-11', DATE'2013-04-30', 9, 'Administrator' FROM DUAL UNION ALL
SELECT 2, 0, DATE'2013-04-30', DATE'2015-10-27', 6, 'Manager' FROM DUAL UNION ALL
SELECT 3, 1, DATE'2014-11-04', NULL, 10, 'CEO' FROM DUAL UNION ALL
SELECT 4, 1, DATE'2017-06-13', NULL, 13, 'CTO' FROM DUAL UNION ALL
SELECT 5, 1, DATE'2010-04-22', DATE'2018-10-22', 1, 'Manager' FROM DUAL UNION ALL
SELECT 6, 1, DATE'2019-08-26', NULL, 2, 'Trainer' FROM DUAL UNION ALL
SELECT 7, 1, DATE'2006-02-27', DATE'2016-11-30', 6, 'Video Creator' FROM DUAL UNION ALL
SELECT 8, 2, DATE'2012-04-30', DATE'2013-11-24', 5, 'CEO' FROM DUAL UNION ALL
SELECT 9, 2, DATE'2013-01-10', NULL, 0, 'CEO' FROM DUAL UNION ALL
SELECT 10, 2, DATE'2010-11-01', DATE'2017-01-01', 7, 'Video Creator' FROM DUAL UNION ALL
SELECT 11, 2, DATE'2010-10-03', DATE'2018-03-01', 8, 'Commentator' FROM DUAL
See the demo.

Answer 2

You columns do not have names in the CTE. Define it with names:
WITH exp_(id_, company, start_, end_, usr, position_) AS (
Here is a db<>fiddle.

Answer 3

You need to give aliases to your columns. It is enough to do it in the first unioned subquery:
INSERT INTO experience (id_, company, start_, end_, usr, position_)
WITH exp_ AS (
SELECT
0 id_,
0 company,
DATE'2013-04-30' start_,
NULL end_,
11 usr,
'CEO' position
FROM DUAL UNION ALL
SELECT 1, 0, DATE'2011-12-11', DATE'2013-04-30', 9, 'Administrator' FROM DUAL UNION ALL
SELECT 2, 0, DATE'2013-04-30', DATE'2015-10-27', 6, 'Manager' FROM DUAL UNION ALL
SELECT 3, 1, DATE'2014-11-04', NULL, 10, 'CEO' FROM DUAL UNION ALL
SELECT 4, 1, DATE'2017-06-13', NULL, 13, 'CTO' FROM DUAL UNION ALL
SELECT 5, 1, DATE'2010-04-22', DATE'2018-10-22', 1, 'Manager' FROM DUAL UNION ALL
SELECT 6, 1, DATE'2019-08-26', NULL, 2, 'Trainer' FROM DUAL UNION ALL
SELECT 7, 1, DATE'2006-02-27', DATE'2016-11-30', 6, 'Video Creator' FROM DUAL UNION ALL
SELECT 8, 2, DATE'2012-04-30', DATE'2013-11-24', 5, 'CEO' FROM DUAL UNION ALL
SELECT 9, 2, DATE'2013-01-10', NULL, 0, 'CEO' FROM DUAL UNION ALL
SELECT 10, 2, DATE'2010-11-01', DATE'2017-01-01', 7, 'Video Creator' FROM DUAL UNION ALL
SELECT 11, 2, DATE'2010-10-03', DATE'2018-03-01', 8, 'Commentator' FROM DUAL
)
SELECT * FROM exp_;
For your use case, you could also consider using the insert all syntax, which avoids the multiple union alls:
insert all
into experience (id_, company, start_, end_, usr, position_)
values(0, 0, date '2013-04-30', null, 11, 'CEO')
into experience (id_, company, start_, end_, usr, position_)
values(1, 0, date '2011-12-11', date '2013-05-30', 9, 'Administrator')
...
select 1 from dual