LeetCode176——第二高的薪水

題目描述

編寫一個 SQL 查詢，獲取 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查詢應該返回 200 做爲第二高的薪水。若是不存在第二高的薪水，那麼查詢應返回 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 過濾掉相同薪水

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

1. 當不存在第二高的薪水時，會返回空而不是 null，作個是否爲 null 的判斷

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

能夠簡寫爲

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