题目：请你获取薪水第二多的员工的emp_no以及其对应的薪水salary，
若有多个员工的薪水为第二多的薪水，则将对应的员工的emp_no和salary全部输出，并按emp_no升序排序。
mysql语句：
drop table if exists salaries ;
CREATE TABLE salaries (emp_no int(11) NOT NULL,salary int(11) NOT NULL,from_date date NOT NULL,to_date date NOT NULL,
PRIMARY KEY (emp_no,from_date));
INSERT INTO salaries VALUES(10001,88958,'2002-06-22','9999-01-01');
INSERT INTO salaries VALUES(10002,72527,'2001-08-02','9999-01-01');
INSERT INTO salaries VALUES(10003,43311,'2001-12-01','9999-01-01');
SELECT a.emp_no, a.salary -- 再套一层查询
FROM(
SELECT emp_no, salary, dense_rank()over(ORDER BY salary DESC)rank_num
FROM salaries
WHERE to_date = '9999-01-01') as a
WHERE a.rank_num = 2;
报错如下：
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '(ORDER BY salary DESC)rank_num
FROM salaries
WHERE to_date = '9999-01-01')a
WHER' at line 3
请问怎么改这个报错o(╥﹏╥)o，非常感谢
