方法一：
select
t0.emp_no
from
(
select
emp_no
from employees
)t0
left join
(
select
emp_no
from dept_manager
)t1
on t0.emp_no = t1.emp_no
where t1.emp_no is null ;
方法二：
select
emp_no
from
employees
where emp_no not in(
select
emp_no
from dept_manager
);