如何获取同一个表中两个字段数据的并集

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 distinct num
  from (
  select distinct fstuid as num
  from tgrade
  union all
  select distinct scourseid as num from tgrade)

select distinct fstuid as num
  from tgrade
  union 
  select distinct scourseid as num from tgrade 直接这样就可以了吧。

如下即可:

select fstuid num from tgrade
union
select scourseid num from tgrade

或者:

select distinct fstuid num from tgrade
union
select distinct scourseid num from tgrade