LeetCode176——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 做为第二高的薪水。若是不存在第二高的薪水，那么查询应返回 null。

+---------------------+
| SecondHighestSalary |
+---------------------+
| 200                 |
+---------------------+

思路

1. 排序，取出排名第二的值

select Salary from Employee
  order by Salary desc
  limit 1, 1;

1. group by 过滤掉相同薪水

select Salary from Employee
  group by Salary
  order by Salary desc
  limit 1, 1;

1. 当不存在第二高的薪水时，会返回空而不是 null，作个是否为 null 的判断

select
  ifnull(
    (select Salary from Employee group by Salary order by Salary desc limit 1, 1),
    null
  ) as SecondHighestSalary;

能够简写为

select 
  (select Salary from Employee group by Salary order by Salary desc limit 1, 1)
  as SecondHighestSalary;