Leetcode No.177 第N高的薪水

+----+--------+
| Id | Salary |
+----+--------+
| 1  | 100    |
| 2  | 200    |
| 3  | 300    |
+----+--------+

+------------------------+
| getNthHighestSalary(2) |
+------------------------+
| 200                    |
+------------------------+

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
  RETURN (
      # Write your MySQL query statement below.
      select distinct(salary)
      from(
            select salary,dense_rank()over(order by Salary desc) as rn
            from Employee
      )a 
      where a.rn=N
  );
END