题解 | #获取所有非manager的员工emp_no#

1.使用IN：
SELECT emp_no FROM empolyees
WHERE emp_no NOT IN (SELECT emp_no FROM dept_manager);

2.使用EXISTS:
SELECT e.emp_no FROM employees AS e
WHERE NOT EXISTS (SELECT d.emp_no FROM dept_manager AS d
WHERE e.emp_no = d.emp_no);
3.根据NULL条件查找
SELECT e.emp_no FROM empployees AS e
LEFT JOIN dept_manager as d
ON e.emp_no = d.emp_no
WHERE dept_no is NULL;