select
c.emp_no,
c.birth_date,
c.first_name,
c.last_name,
c.gender,
c.hire_date
from(
select a.emp_no,
    a.birth_date,
    a.first_name,
    a.last_name,
    a.gender,
    a.hire_date,
    b.dept_no
from employees a
left join dept_emp b
on a.emp_no=b.emp_no
) c
where c.dept_no is null
或
select *
from employees a
where not exists (select b.emp_no from dept_emp b where b.emp_no=a.emp_no)