SQL 前几名的问题

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。

题解：
** 解1：看到力扣一个大佬的解答，觉得很有收获。下面是他的思路：对于这种分组内取前几名的问题，可以先group by然后用having count()来筛选，比如这题，找每个部门的工资前三名，那么先在子查询中用Employee和自己做连接，连接条件是【部门相同但是工资比我高】，那么接下来按照having count(Salary) <= 2来筛选的原理是：如果【跟我一个部门而且工资比我高的人数】不超过2个，那么我一定是部门工资前三，这样内层查询可以查询出所有符合要求的员工ID，接下来外层查询就简单了。**

SELECT d.Name AS Department, e.Name AS Employee, e.Salary FROM Employee e JOIN Department d ON
d.Id = e.DepartmentId
WHERE e.Id IN  
(SELECT e1.Id FROM Employee e1 LEFT JOIN Employee e2 ON e1.DepartmentId = e2.DepartmentId AND e2.Salary > e1.Salary
GROUP BY e1.Id 
HAVING COUNT(DISTINCT e2.Salary) <= 2)
ORDER BY e.Salary DESC

解2：

select department.name as Department,a.name as Employee,Salary
from (
   select *, 
          dense_rank() over (partition by departmentid
                       order by salary desc) as ranking
   from employee) as a join department on department.id = a.departmentid
where ranking <= 3
ORDER BY salary desc

这里可以得到解topn的通用思路：
topN问题 sql模板

select *
from (
   select *, 
          row_number() over (partition by 要分组的列名
                       order by 要排序的列名 desc) as ranking
   from 表名) as a
where ranking <= N

其中窗口函数要根据具体的题目需求选择row_number或rank或dense_rank
来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/department-top-three-salaries
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