mysql查询每个部门/班级前几名

Employee 表包含所有员工信息，每个员工有其对应的 Id, salary 和 department Id 。

+----+-------+--------+--------------+
| Id | Name  | Salary | DepartmentId |
+----+-------+--------+--------------+
| 1  | Joe   | 70000  | 1            |
| 2  | Henry | 80000  | 2            |
| 3  | Sam   | 60000  | 2            |
| 4  | Max   | 90000  | 1            |
| 5  | Janet | 69000  | 1            |
| 6  | Randy | 85000  | 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，找出每个部门工资前三高的员工。例如，根据上述给定的表格，查询结果应返回：

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

select d.Name Department,e1.Name Employee,e1.Salary
from Employee e1,Department d where e1.DepartmentId=d.Id and
(select count(distinct e2.Salary) from Employee e1,Employee e2 where e1.DepartmentId=e2.DepartmentId and e2.Salary>e1.Salary)<3
order by d.Name,e1.Salary DESC

意思就是：我们要查询的这个人，在这部门中工资比他高的少于3个人（0,1,2人）。
括号里面是查询在这部门中工资比他高的人的数量。