CREATE TABLE A (num NUMBER, name VARCHAR2(10), grade varchar2(1), marks NUMBER(10));

 INSERT INTO A VALUES(1, 'AAKSHAY','C',20) ;
 commit;
 

SQL> select dummy from dual
  2  union all
  3  select dummy from dual
  4  /
D
-
X
X

2 rows selected.

SQL> select dummy
  2  from dual,
  3       (select 1 l from dual union all select 2 from dual)
  4  /
D
-
X
X

2 rows selected.