1、176题目，Second Highest Salary，https://leetcode.com/problems/second-highest-salary/#/description

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

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

　　For example, given the above Employee table, the query should return 200 as the second highest salary. If there is no second highest salary, then the query should return null.

+---------------------+
| SecondHighestSalary |
+---------------------+
| 200                 |
+---------------------+

select (
  select distinct Salary from Employee order by Salary Desc limit 1 offset 1
)as second；
————————————————————————————————————————
177. Nth Highest Salary

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

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

CREATE FUNCTION getNthHighestSalary(N INT)

RETURNS INT

BEGIN

DECLARE M INT;

SET M=N-1;

　　RETURN (

　　　　SELECT DISTINCT Salary FROM Employee ORDER BY Salary DESC LIMIT M, 1]

);

END

leetcode_sql_1