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                 |
+---------------------+

解題思路：

注意兩點：首先如果沒有第二高的工資，應該返回NULL，如果直接使用查詢會查詢為空，而不會返回NULL，所以需要將此查詢作為子查詢。

第二點是找第二高的工資，是工資多少排第二，而不是工資排名第二，所以需要distinct修飾。

select 
(select distinct Salary from Employee order by Salary desc limit 1,1)
as SecondHighestSalary;