1. 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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2. 解题
# Write your MySQL query statement below
select Department, Employee, Salary
from
(
select d.Name Department, e.Name Employee, Salary,
dense_rank() over(partition by d.Name order by Salary desc) rnk
from Employee e left join Department d
on e.DepartmentId = d.Id
) t
where rnk <= 3 and Department is not null
or
# Write your MySQL query statement below
select Department, Employee, Salary
from
(
select d.Name Department, e.Name Employee, Salary,
dense_rank() over(partition by d.Name order by Salary desc) rnk
from Employee e right join Department d
on e.DepartmentId = d.Id
) t
where rnk <= 3 and Employee is not null
or 使用内连接，自动去除 null 的行
select Department, Employee, Salary
from
(
select d.Name Department, e.Name Employee, Salary,
dense_rank() over(partition by d.Name order by Salary desc) rnk
from Employee e join Department d
on e.DepartmentId = d.Id
) t
where rnk <= 3
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