LeetCode.177 Nth Highest Salary

题目：

Write a SQL query to get the n^th highest salary from the Employee table.

+----+--------+
| Id | Salary |
+----+--------+
| 1  | 100    |
| 2  | 200    |
| 3  | 300    |
+----+--------+

For example, given the above Employee table, the n^th highest salary wheren = 2 is200. If there is no n^th highest salary, then the query should returnnull.

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

分析：

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
  DECLARE M INT;
  SET M=N-1;
  RETURN (
      #找出第N大的数据,使用limit偏移求解
      # Write your MySQL query statement below.
      SELECT DISTINCT Salary FROM Employee ORDER BY Salary DESC LIMIT M,1
  );
END