部门工资最高的员工

问题描述

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  |
+------------+----------+--------+

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/department-highest-salary

思路

方法：使用 JOIN 和 IN 语句

算法
因为 Employee 表包含 Salary 和 DepartmentId 字段，我们可以以此在部门内查询最高工资。

SELECT
    DepartmentId, MAX(Salary)
FROM
    Employee
GROUP BY DepartmentId;

注意：有可能有多个员工同时拥有最高工资，所以最好在这个查询中不包含雇员名字的信息。

| DepartmentId | MAX(Salary) |
|--------------|-------------|
| 1            | 90000       |
| 2            | 80000       |

然后，我们可以把表 Employee 和 Department 连接，再在这张临时表里用 IN 语句查询部门名字和工资的关系。

代码见题解

| Department | Employee | Salary |
|------------|----------|--------|
| Sales      | Henry    | 80000  |
| IT         | Max      | 90000  |

题解

SELECT
    Department.name AS 'Department',
    Employee.name AS 'Employee',
    Salary
FROM
    Employee
        JOIN
    Department ON Employee.DepartmentId = Department.Id
WHERE
    (Employee.DepartmentId , Salary) IN
    (   SELECT
            DepartmentId, MAX(Salary)
        FROM
            Employee
        GROUP BY DepartmentId
    )
;

作者：LeetCode
链接：https://leetcode-cn.com/problems/two-sum/solution/bu-men-gong-zi-zui-gao-de-yuan-gong-by-leetcode/