查找第二高的薪水

地址:https://leetcode-cn.com/problems/second-highest-salary/solution/di-er-gao-de-xin-shui-by-leetcode/

## 编写一个 SQL 查询，获取 Employee 表中第二高的薪水（Salary） 。     
     示例：
    +----+--------+
    | Id | Salary |
    +----+--------+
    | 1  | 100    |
    | 2  | 200    |
    | 3  | 300    |
    +----+--------+
    例如上述 Employee 表，SQL查询应该返回 200 作为第二高的薪水。如果不存在第二高的薪水，那么查询应返回 null。
    
    +---------------------+
    | SecondHighestSalary |
    +---------------------+
    | 200                 |
    +---------------------+
   
`解题思路`
1 limit 或者OFFSET
       select IFNULL((select distinct(Salary) 
         from Employee
         order by Salary desc
         limit 1,1),null) as SecondHighestSalary
     
2   嵌套表查询
        `select max(Salary) SecondHighestSalary
         from employee
         where
         salary<(select max(salary) from employee)`