解题思路1：
SELECT emp_no,salary from salaries
where to_date='9999-01-01'
order by salary DESC LIMIT 1,1

解题思路2：
SELECT emp_no,salary from
(
SELECT emp_no,salary,ROW_NUMBER() over(order by salary desc) t
FROM salaries
WHERE to_date='9999-01-01'
) as new
where new.t=2