题目：获取所有非manager的员工的emp_no

方法1：NOT IN+子查询
select emp_no
from employees
where emp_no not in (select emp_no from dept_manager);

方法2：LEFT JOIN左连接+IS NULL
SELECT e.emp_no
FROM employees AS e
LEFT JOIN dept_manager AS d
ON e.emp_no=d.emp_no
WHERE dept_no IS NULL;