SQL再谈分组topN

题目

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
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题解

这题目的前三有所不同，是工资的前三，而不是前三位员工，

# Write your MySQL query statement below

select 
    a.Name as Department,
    b.Name as Employee,
    b.Salary as Salary
from    
	Department a
join
(
    select
        c.DepartmentId,
        c.Name,
        c.Salary
    from
        Employee c
    left join
        Employee d
    on c.DepartmentId = d.DepartmentId and c.Salary < d.Salary
    group by c.Id
    having count(distinct d.Salary) < 3
    order by c.DepartmentId, c.Salary desc
) b
on a.Id = b.DepartmentId;

如果是找前三位员工，允许并列第三的话：

# Write your MySQL query statement below

select 
    a.Name as Department,
    b.Name as Employee,
    b.Salary as Salary
from
    Department a
join
(
    select
        c.DepartmentId,
        c.Name,
        c.Salary
    from
        Employee c
    left join
        Employee d
    on c.DepartmentId = d.DepartmentId and c.Salary < d.Salary
    group by c.Id
    having count(d.Id) < 3
    order by c.DepartmentId, c.Salary desc
) b
on a.Id = b.DepartmentId;