# 方法1:子查询
# select emp_no
# from employees
# where emp_no not in (
#   select distinct emp_no
#   from dept_manager)

# 方法2:连接 (效率更高)
# SELECT e.emp_no
# from employees e
# left join dept_manager d
# on e.emp_no=d.emp_no
# where dept_no is null