本题考查两表的差集，使用子查询最为简单，使用左外连接后，判断非manager员工emp_no为空即：IS NULL

# 左外连接+IS NULL
# SELECT e.emp_no
# FROM employees AS e
# LEFT OUTER JOIN dept_manager AS d
# ON e.emp_no = d.emp_no
# WHERE dept_no IS NULL
# 子查询
SELECT e.emp_no
FROM employees AS e
WHERE e.emp_no NOT IN (SELECT d.emp_no FROM dept_manager AS d)