176. 第二高的薪水

题目描述

编写一个 SQL 查询，获取 Employee 表中第二高的薪水（Salary） 。

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

分析

select distinct salary from Employee order by salary desc limit 1,1 就可以，但是输不出null，所以外面再加一层 select (select distinct salary from Employee order by salary desc limit 1,1) as SecondHighestSalary ;
当Employee表里只有一条数据时，内层SQL语句查询不到数据，其返回结果是空，而外层SQL的作用是把内层的查询结果赋值给SecondHighestSalary.

贴出代码

select 
(select distinct salary 
from Employee 
 order by salary 
 desc limit 1,1) 
 as SecondHighestSalary ;