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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解答：
先找出公司工资前三的员工，不超过三个的工资比这些值高
mysql：
select e1.Name as 'Employee', e1.Salary
from Employee e1
where 3 >
(
select count(distinct e2.Salary)
from Employee e2
where e2.Salary > e1.Salary
)
;
举例：
当
e1 = e2 = [1,2,3,4,5]
e1.Salary = 1，e2.Salary 可以取值 [2,3,4,5]，count(DISTINCT e2.Salary) = 4
e1.Salary = 2，e2.Salary 可以取值 [3,4,5]，count(DISTINCT e2.Salary) = 3
e1.Salary = 3，e2.Salary 可以取值 [4,5]，count(DISTINCT e2.Salary) = 2
e1.Salary = 4，e2.Salary 可以取值 [5]，count(DISTINCT e2.Salary) = 1
e1.Salary = 5，e2.Salary 可以取值 []，count(DISTINCT e2.Salary) = 0
最后 3 > count(DISTINCT e2.Salary)，所以 e1.Salary 可取值为 [3,4,5]，即集合前 3 高的薪水
再把表 Department 和表 Employee 连接，获得各个部门工资前三高的员工。
mysql:
select d.Name as Department,e1.Name as Employee,e1.Salary
from Employee e1
join Department d
on e1.DepartmentId = d.Id
where
3 > (select
count(distinct e2.Salary)
from Employee e2
where e2.Salary > e1.Salary
and e1.DepartmentId = e2.DepartmentId)