SQL> SELECT deptno
  2  FROM   emp
  3  ,      dept
  4  WHERE  ROWNUM = 1
  5  /
SELECT deptno
       *
ERROR at line 1:
ORA-00918: column ambiguously defined
 
 
SQL> SELECT <font color=red>e</font>.deptno
  2  FROM   emp          <font color=red>e</font>
  3  ,      dept         <font color=red>d</font>
  4  WHERE  ROWNUM = 1
  5  /
 
    DEPTNO
----------
        20
 
SQL>