【leetcode】Second Highest Salary

原题如下：

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 second highest salary is 200. If there is no second highest salary, then the query should return null.

# Write your MySQL query statement below
select max(Salary)
from Employee where Salary<(select max(Salary) from Employee )