编写一个 SQL 查询，获取 Employee 表中第 n 高的薪水（Salary）。
+----+--------+
| Id | Salary |
+----+--------+
| 1 | 100 |
| 2 | 200 |
| 3 | 300 |
+----+--------+
例如上述 Employee 表，n = 2 时，应返回第二高的薪水 200。如果不存在第 n 高的薪水，那么查询应返回 null。
+------------------------+
| getNthHighestSalary(2) |
+------------------------+
| 200 |
+------------------------+
CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
SET N := N-1;
RETURN (
# Write your MySQL query statement below.
select ifnull((select distinct Salary as SecondHighestSalary
from Employee
order by Salary DESC
limit 1 offset N),null) as SecondHighestSalary
);
END