一个sql查询问题

create table tgrade (
fstuid varchar2(10),
scourseid varchar2(10),
f_grade int null,
Primary Key(fstuid,scourseid)
);

 
 insert into tgrade values ('1', '2', 1);
 insert into tgrade values ('2', '3', 1);
 insert into tgrade values ('3', '4', 1);
 insert into tgrade values ('4', '5', 1);

如何过滤出fstuid和scourseid的并集，即1，2，3，4，5
我写的sql为什么不起作用
select num
  from (
  select distinct fstuid as num
  from tgrade
  union all
  select distinct scourseid as num from tgrade)

SELECT num
  from (
  select distinct fstuid as num
  from tgrade
  UNION
  select distinct scourseid as num from tgrade)
  ORDER BY NUM;

注意一下union 和 union all 的区别
union all不会去重复行