lc-177

表: Employee

+-------------+------+
| Column Name | Type |
+-------------+------+
| id          | int  |
| salary      | int  |
+-------------+------+
在 SQL 中，id 是该表的主键。
该表的每一行都包含有关员工工资的信息。

查询 Employee 表中第 n 高的工资。如果没有第 n 个最高工资，查询结果应该为 null 。

查询结果格式如下所示。

示例 1:

输入: 
Employee table:
+----+--------+
| id | salary |
+----+--------+
| 1  | 100    |
| 2  | 200    |
| 3  | 300    |
+----+--------+
n = 2
输出: 
+------------------------+
| getNthHighestSalary(2) |
+------------------------+
| 200                    |
+------------------------+

示例 2:

输入: 
Employee 表:
+----+--------+
| id | salary |
+----+--------+
| 1  | 100    |
+----+--------+
n = 2
输出: 
+------------------------+
| getNthHighestSalary(2) |
+------------------------+
| null                   |
+------------------------+

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