LeeCode——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                 |
+---------------------+

此题的难点是：

• 针对可能存在的重复数值，选出唯一的数值(使用distinct)；
• 选取第二个数值，则需要使用limit,offset确定数值；
• 查询的数值可能为不存在，此时就需要返回NULL(使用IFNULL).

答题如下所示：

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