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

思路：

1、如果结果为空的话返回null，不为空则返回结果，这个条件可以用mysql的ISNULL函数来实现

2、结果仅为一条记录，需要用到分页关键字：limit,这里的条件设置为 limit 0,1（注意是0到1）；

3、最后的记录名为SecondHighestSalary，我们在查询语句的最后需要加上一句：as SecondHighestSalary

4、第二高的薪水，我们可以通过将 select salary where salary < (select max(salary) form Employee)这个语句来获得所有比最大薪水小的薪水值，然后再通过order by desc来进行降序排列，然后用limit关键字来实现就行啦。

最后的结果如下：

# Write your MySQL query statement below

select IFNULL((

SELECT salary from employee  where salary < (select max(salary) from employee) ORDER BY salary DESC LIMIT 0,1),

NULL) as SecondHighestSalary