Second Highest Salary

PROBLEM：

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

SOLVE：

# Write your MySQL query statement below
select (
    select distinct Salary 
    from Employee 
    order by Salary Desc 
    limit 1 , 1
)as SecondHighestSalary

# Write your MySQL query statement below
SELECT max(Salary) AS SecondHighestSalary
FROM Employee
WHERE Salary < (SELECT max(Salary) 
                FROM Employee)

分析：即使表中沒有數據，最後也會返回null。原理，我也不知道！