Nth Highest Salary
題意：
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.
解題思路：
需要注意的就是我們先定義一個變數m，表示取出的資料中，要跳過M個之後，再選1個出來，即是我們要的數。
LIMIT N-1, 1 ：表示跳過n-1個數之後１挑1個數出來。
CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
DECLARE M INT;
SET M = N - 1;
RETURN (
# Write your MySQL query statement below.
SELECT DISTINCT Salary FROM Employee ORDER BY Salary DESC LIMIT M, 1
);
END