Sql 选择同一表中的记录子集

有一个表显示权限和某些对象的映射，即对象“obj1”具有“perm1”和“perm2”权限，“obj2”-“perm1”和“perm3”

Permission_id Object_Id
  perm1      |   obj1
  perm2      |   obj1
  perm1      |   obj2
  perm3      |   obj2

问题是：如何获得应用于所有对象的权限子集？即：

Permission_id 
    perm1     

---

以下使用计数和聚合来实现此目的：

select permission_id
from permissionobjects po
group by permission_id
having count(distinct object_id) = (select count(distinct object_id) from permissionobjects);