Your Query

select dname from dept
where deptno = (select deptno from room
where roomno = (select roomno from emp
where deptno is NULL));

Query Result

  dname
----------
 Database
(1 row)

Your Query

select dname from dept
    where deptno = (select distinct a.deptno from dept a
    except select distinct d.deptno from
    (select distinct b.eqname, c.deptno from equip b,
    (select distinct deptno from equip) c
        except select * from equip) d);

Query Result

  dname
----------
 Database
(1 row)