Write a SQL query to get the nth highest salary from the Employee table.

+----+--------+
| Id | Salary |
+----+--------+
| 1  | 100    |
| 2  | 200    |
| 3  | 300    |
+----+--------+
For example, given the above Employee table, the nth highest salary where n = 2 is 200. If there is no nth highest salary, then the query should return null.

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

此题相较于Second Highest Salary做了一些改进：

• 创建mysql function;
• 需要判断传入参数的合理性.

因此，对代码改动如下所示：

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
  DECLARE P INT DEFAULT N-1;
  IF (P<0) THEN
    RETURN NULL;
  ELSE
  RETURN (
      # Write your MySQL query statement below.
      SELECT IFNULL(
            (
                SELECT DISTINCT Salary 
                FROM Employee 
                ORDER BY Salary DESC 
                LIMIT P,1)
            ,NULL)
          AS SecondHighestSalary   
  );
  END IF;
END