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部门工资前三高的所有员工

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这是一道Leetcode数据库部分的Hard题,本文决定采用最简单的方式来一步一步分解

题目:部门工资前三高的所有员工

Employee 表包含所有员工信息，每个员工有其对应的工号 Id，姓名 Name，工资 Salary 和部门编号 DepartmentId 。

+----+-------+--------+--------------+
| Id | Name  | Salary | DepartmentId |
+----+-------+--------+--------------+
| 1  | Joe   | 85000  | 1            |
| 2  | Henry | 80000  | 2            |
| 3  | Sam   | 60000  | 2            |
| 4  | Max   | 90000  | 1            |
| 5  | Janet | 69000  | 1            |
| 6  | Randy | 85000  | 1            |
| 7  | Will  | 70000  | 1            |
+----+-------+--------+--------------+

Department 表包含公司所有部门的信息。

+----+----------+
| Id | Name     |
+----+----------+
| 1  | IT       |
| 2  | Sales    |
+----+----------+

编写一个 SQL 查询，找出每个部门获得前三高工资的所有员工。例如，根据上述给定的表，查询结果应返回：

+------------+----------+--------+
| Department | Employee | Salary |
+------------+----------+--------+
| IT         | Max      | 90000  |
| IT         | Randy    | 85000  |
| IT         | Joe      | 85000  |
| IT         | Will     | 70000  |
| Sales      | Henry    | 80000  |
| Sales      | Sam      | 60000  |
+------------+----------+--------+

解释：

IT 部门中，Max 获得了最高的工资，Randy 和 Joe 都拿到了第二高的工资，Will 的工资排第三。销售部门（Sales）只有两名员工，Henry 的工资最高，Sam 的工资排第二。

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/department-top-three-salaries 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

Create table If Not Exists Employee (Id int, Name varchar(255), Salary int, DepartmentId int);
Create table If Not Exists Department (Id int, Name varchar(255));
Truncate table Employee;
insert into Employee (Id, Name, Salary, DepartmentId) values ('1', 'Joe', '85000', '1');
insert into Employee (Id, Name, Salary, DepartmentId) values ('2', 'Henry', '80000', '2');
insert into Employee (Id, Name, Salary, DepartmentId) values ('3', 'Sam', '60000', '2');
insert into Employee (Id, Name, Salary, DepartmentId) values ('4', 'Max', '90000', '1');
insert into Employee (Id, Name, Salary, DepartmentId) values ('5', 'Janet', '69000', '1');
insert into Employee (Id, Name, Salary, DepartmentId) values ('6', 'Randy', '85000', '1');
insert into Employee (Id, Name, Salary, DepartmentId) values ('7', 'Will', '70000', '1');
Truncate table Department;
insert into Department (Id, Name) values ('1', 'IT');
insert into Department (Id, Name) values ('2', 'Sales');

题解

进行三步分解

  1. 先对每个部门的工资进行排行
  2. 取出每个部门工资前三高的人的信息
  3. 使用Department.Name代替Employee.DepartmentID

先对每个部门的工资进行排行

我们先理解什么是排名,排名就是看看比你多的工资有多少个人,比如说总共有工资档次:200 100 50 20要取50的排名就是看看大于等于50一共有几个档次,这里我们看到有3个档次200 150 50,所以你的排名就是第三名.所以我们可以按照这个意思来写SQL

select DepartmentId,
       Name,
       `Salary`,
       (select count(distinct `Salary`) from `Employee` b
        where b.Salary >= a.Salary
          and a.`DepartmentId` = b.DepartmentId
       ) as `Rank`
from `Employee` a

查出的结果集为

+--------------+-------+--------+------+
| DepartmentId | Name  | Salary | Rank |
+--------------+-------+--------+------+
| 1            | Joe   | 85000  | 2    |
| 2            | Henry | 80000  | 1    |
| 2            | Sam   | 60000  | 2    |
| 1            | Max   | 90000  | 1    |
| 1            | Janet | 69000  | 4    |
| 1            | Randy | 85000  | 2    |
| 1            | Will  | 70000  | 3    |
+--------------+-------+--------+------+

-- 按照 DepartmentId,Rank 排序来看就是
+--------------+-------+--------+------+
| DepartmentId | Name  | Salary | Rank |
+--------------+-------+--------+------+
| 1            | Max   | 90000  | 1    |
| 1            | Randy | 85000  | 2    |
| 1            | Joe   | 85000  | 2    |
| 1            | Will  | 70000  | 3    |
| 1            | Janet | 69000  | 4    |
| 2            | Henry | 80000  | 1    |
| 2            | Sam   | 60000  | 2    |
+--------------+-------+--------+------+

取出每个部门工资前三高的人的信息

这里很明显就是写一个where条件

select *
from (
    select DepartmentId,
       Name,
       `Salary`,
       (select count(distinct `Salary`) from `Employee` b
        where b.Salary >= a.Salary
          and a.`DepartmentId` = b.DepartmentId
       ) as `Rank`
	from `Employee` a
    )a
where 
	`rank` <= 3

查出来的结果集为

+--------------+-------+--------+------+
| DepartmentId | Name  | Salary | Rank |
+--------------+-------+--------+------+
| 1            | Joe   | 85000  | 2    |
| 2            | Henry | 80000  | 1    |
| 2            | Sam   | 60000  | 2    |
| 1            | Max   | 90000  | 1    |
| 1            | Randy | 85000  | 2    |
| 1            | Will  | 70000  | 3    |
+--------------+-------+--------+------+

-- 按照 DepartmentId,Rank 排序来看就是

+--------------+-------+--------+------+
| DepartmentId | Name  | Salary | Rank |
+--------------+-------+--------+------+
| 1            | Max   | 90000  | 1    |
| 1            | Randy | 85000  | 2    |
| 1            | Joe   | 85000  | 2    |
| 1            | Will  | 70000  | 3    |
| 2            | Henry | 80000  | 1    |
| 2            | Sam   | 60000  | 2    |
+--------------+-------+--------+------+

使用Department.Name代替Employee.DepartmentID使用Department.Name代替Employee.DepartmentID

这里可以看到只要连表查一下就可以了,这个代码应该不难

select b.`Name` as `Department`, a.Name as Employee,Salary 
from (
    select 
    	DepartmentId,
    	Name,
    	`Salary`,
   		(
            select 
            	count(distinct `Salary`) 
            from 
            	`Employee` b 
            where 
            	b.Salary>=a.Salary and a.`DepartmentId` = b.DepartmentId
        ) as `Rank` 
    from `Employee` a
)a left join `Department` b on a.DepartmentId = b.id 
where 
	`rank` <= 3 
	and 
	b.`name` is not null;
	
-- 这里 b.`name` is not null 不是一个必须的,是为了判断如果Department表没有这个部门就不算记录.

最后代码

select b.`Name` as `Department`, a.Name as Employee,Salary 
from (
    select 
    	DepartmentId,
    	Name,
    	`Salary`,
   		(
            select 
            	count(distinct `Salary`) 
            from 
            	`Employee` b 
            where 
            	b.Salary>=a.Salary and a.`DepartmentId` = b.DepartmentId
        ) as `Rank` 
    from `Employee` a
)a left join `Department` b on a.DepartmentId = b.id 
where 
	`rank` <= 3 
	and 
	b.`name` is not null;
	
-- 这里 b.`name` is not null 不是一个必须的,是为了判断如果Department表没有这个部门就不算记录.
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