Order the two tables using nulls first

orderEmpNulls

select empno
,      ename
,      job
,      deptno
from( 
   select null as empno
   ,      null as ename
   ,      null as job
   ,      null as deptno
   from   emp
   union
   select empno
   ,      ename
   ,      job
   ,      deptno
   from   emp
   where deptno = 10
)
order by empno nulls first