/**
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
  RETURN (
      # Write your MySQL query statement below.
      SELECT DISTINCT e.salary
      FROM employee e
      WHERE (SELECT count(DISTINCT salary) FROM employee WHERE salary>e.salary) = N-1
  );
END