Question

176. 第二高的薪水

English Version

题目描述

Employee 表：

+-------------+------+
| Column Name | Type |
+-------------+------+
| id      | int  |
| salary    | int  |
+-------------+------+
在 SQL 中，id 是这个表的主键。
表的每一行包含员工的工资信息。

 

查询并返回 Employee 表中第二高的薪水 。如果不存在第二高的薪水，查询应该返回 null(Pandas 则返回 None) 。

查询结果如下例所示。

 

示例 1：

输入：
Employee 表：
+----+--------+
| id | salary |
+----+--------+
| 1  | 100  |
| 2  | 200  |
| 3  | 300  |
+----+--------+
输出：
+---------------------+
| SecondHighestSalary |
+---------------------+
| 200         |
+---------------------+

示例 2：

输入：
Employee 表：
+----+--------+
| id | salary |
+----+--------+
| 1  | 100  |
+----+--------+
输出：
+---------------------+
| SecondHighestSalary |
+---------------------+
| null        |
+---------------------+

解法

方法一：使用 LIMIT 语句和子查询

我们可以按照薪水降序排列，然后使用 LIMIT 语句来获取第二高的薪水，如果不存在第二高的薪水，那么就返回 null。

import pandas as pd


def second_highest_salary(employee: pd.DataFrame) -> pd.DataFrame:
  # Drop any duplicate salary values to avoid counting duplicates as separate salary ranks
  unique_salaries = employee["salary"].drop_duplicates()

  # Sort the unique salaries in descending order and get the second highest salary
  second_highest = (
    unique_salaries.nlargest(2).iloc[-1] if len(unique_salaries) >= 2 else None
  )

  # If the second highest salary doesn't exist (e.g., there are fewer than two unique salaries), return None
  if second_highest is None:
    return pd.DataFrame({"SecondHighestSalary": [None]})

  # Create a DataFrame with the second highest salary
  result_df = pd.DataFrame({"SecondHighestSalary": [second_highest]})

  return result_df

# Write your MySQL query statement below
SELECT
  (
    SELECT DISTINCT salary
    FROM Employee
    ORDER BY salary DESC
    LIMIT 1, 1
  ) AS SecondHighestSalary;

方法二：使用 MAX() 函数和子查询

我们也可以使用 MAX() 函数，从小于 MAX() 的薪水中挑选一个最大的薪水即可。

# Write your MySQL query statement below
SELECT MAX(salary) AS SecondHighestSalary
FROM Employee
WHERE salary < (SELECT MAX(salary) FROM Employee);

方法三：使用 DISTINCT 和窗口函数

我们还可以先通过 DENSE_RANK() 函数计算出每个员工的薪水排名，然后再筛选出排名为 $2$ 的员工薪水即可。

# Write your MySQL query statement below
WITH T AS (SELECT salary, DENSE_RANK() OVER (ORDER BY salary DESC) AS rk FROM Employee)
SELECT (SELECT DISTINCT salary FROM T WHERE rk = 2) AS SecondHighestSalary;