176. Second Highest Salary

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

要注意两个可能出现的情况，第一个当表格里的数据只有一个的时候，此时要返回null, 第二种情况是表格中salary都是同一个值，按照题目就不存在第二个最大值，也返回null.

SELECT 
    (SELECT  DISTINCT Salary  FROM Employee
    ORDER BY Salary Desc
    LIMIT 1, 1) AS SecondHighestSalary;

SELECT 
    IFNULL((SELECT  DISTINCT Salary  FROM Employee
    ORDER BY Salary Desc
    LIMIT 1, 1), NULL) AS SecondHighestSalary;