SQL> select deptno, ename              --> no table alias
  2  from emp e, dept d
  3  where e.deptno = d.deptno;
select deptno, ename
       *
ERROR at line 1:
ORA-00918: column ambiguously defined


SQL> select d.deptno, e.ename          --> using table alias
  2  from emp e, dept d
  3  where e.deptno = d.deptno;

    DEPTNO ENAME
---------- ----------
        20 SMITH
        30 ALLEN
        30 WARD
        20 JONES
        30 MARTIN
        30 BLAKE
        10 CLARK
        20 SCOTT
        10 KING
        30 TURNER
        20 ADAMS
        30 JAMES
        20 FORD
        10 MILLER

14 rows selected.

SQL>