Why does Create table show ORA-00957: duplicate column name - oracle

Question

Does anyone know why this select statement on this query runs just fine but when i add cte as in front of the same select statement it gives this error:
SQL Error: ORA-00957: duplicate column name)
CREATE TABLE t1 AS
SELECT *
FROM NS_F3
LEFT JOIN NS_FA2
ON NS_F3.PI_CANDIDATE_NUM = NS_FA2.PI_CANDIDATE_NUM
WHERE REGEXP_LIKE(NS_F3.TITLE, 'intern($|ship|[^a-z])', 'i');

Answer 1

It is because tables NS_F3 and NS_FA2 contain columns with the same name - in that case, you should use column alias to avoid duplicate column names.
Here's an example: I'm creating a simple table, as extract of several columns from Scott's EMP table:
SQL> create table t_first as select deptno, empno, ename from emp where rownum < 5;
Table created.
Join it to DEPT table:
SQL> select * from t_first e join dept d on e.deptno = d.deptno;
DEPTNO EMPNO ENAME DEPTNO DNAME LOC
---------- ---------- ---------- ---------- -------------- -------------
20 7369 SMITH 20 RESEARCH DALLAS
20 7566 JONES 20 RESEARCH DALLAS
30 7521 WARD 30 SALES CHICAGO
30 7499 ALLEN 30 SALES CHICAGO
^^^^^^^^^ ^^^^^^^^
this is DEPTNO column ... ... and here's another one
As long as it works in SELECT, it won't work in CREATE TABLE:
SQL> create table test as
2 select * from t_first e join dept d on e.deptno = d.deptno;
select * from t_first e join dept d on e.deptno = d.deptno
*
ERROR at line 2:
ORA-00957: duplicate column name
The solution is to use a column alias, such as:
SQL> create table test as
2 select e.deptno emp_deptno, --> first alias
3 e.empno,
4 e.ename,
5 d.deptno, dept_deptno --> second alias
6 d.dname,
7 d.loc
8 from t_first e join dept d on e.deptno = d.deptno;
Table created.
SQL> select * From test;
EMP_DEPTNO EMPNO ENAME DEPT_DEPTNO DNAME LOC
---------- ---------- ---------- ----------- -------------- -------------
20 7369 SMITH 20 RESEARCH DALLAS
20 7566 JONES 20 RESEARCH DALLAS
30 7521 WARD 30 SALES CHICAGO
30 7499 ALLEN 30 SALES CHICAGO
SQL>