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 |
+----+-------+--------+--------------+
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，找出每个部门工资最高的员工。例如，根据上述给定的表格，Max 在 IT 部门有最高工资，Henry 在 Sales 部门有最高工资。
+------------+----------+--------+
| Department | Employee | Salary |
+------------+----------+--------+
| IT | Max | 90000 |
| Sales | Henry | 80000 |
+------------+----------+--------+
题目来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/department-highest-salary
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采用分组 topN 的做法
select a.Name as Department, b.Name as Employee, b.Salary
from Department a join
(
select a.Name, a.Salary, a.DepartmentId
from Employee a
where
(
select count(*)
from Employee b
where a.DepartmentId = b.DepartmentId
and a.Salary < b.Salary
) < 1
) b
on a.Id = b.DepartmentId
方案二
select a.Name as Department, b.Name as Employee, b.Salary
from Department a join
(
select c.Name, c.Salary, c.DepartmentId
from Employee c,
(
select DepartmentId, max(Salary) as Salary
from Employee
group by DepartmentId
) d
where c.DepartmentId = d.DepartmentId
and c.Salary = d.Salary
) b
on a.Id = b.DepartmentId;