数据库SQL-第二高的薪水

编写一个 SQL 查询，获取 Employee 表中第二高的薪水（Salary） 。

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

例如上述 Employee 表，SQL查询应该返回 200 作为第二高的薪水。如果不存在第二高的薪水，那么查询应返回 null。

法一：
SELECT MAX(Salary) AS SecondHighestSalary FROM Employee
WHERE Salary != (SELECT MAX(Salary) FROM Employee)
法二：
SELECT MAX(Salary) AS SecondHighestSalary FROM Employee
WHERE Salary < (SELECT MAX(Salary) FROM Employee)
法三：
select IFNULL((select distinct(Salary) 
from Employee
order by Salary desc
limit 1,1),null) as SecondHighestSalary

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/second-highest-salary