题目

sql架构如下所示

Create table If Not Exists Employee (id int, name varchar(255), salary int, departmentId int)
Create table If Not Exists Department (id int, name varchar(255))
Truncate table Employee
insert into Employee (id, name, salary, departmentId) values ('1', 'Joe', '70000', '1')
insert into Employee (id, name, salary, departmentId) values ('2', 'Jim', '90000', '1')
insert into Employee (id, name, salary, departmentId) values ('3', 'Henry', '80000', '2')
insert into Employee (id, name, salary, departmentId) values ('4', 'Sam', '60000', '2')
insert into Employee (id, name, salary, departmentId) values ('5', 'Max', '90000', '1')
Truncate table Department
insert into Department (id, name) values ('1', 'IT')
insert into Department (id, name) values ('2', 'Sales')

题目如下所示

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

Department 表包含公司所有部门的信息。

编写一个 SQL 查询，找出每个部门工资最高的员工。对于上述表，您的 SQL 查询应返回以下行（行的顺序无关紧要）。

解释：

Max 和 Jim 在 IT 部门的工资都是最高的，Henry 在销售部的工资最高。

思路 - group by 加 in

(Employee.DepartmentId , Salary) IN 代码模块 可以这样使用
本身要查看最大最也就是要分组
而且两个表要字段相同可以通过id相等进行匹配

难点在于要和in进行结合，具体可看看这部分的代码

# Write your MySQL query statement below
SELECT
	Department.NAME AS Department,
	Employee.NAME AS Employee,
	Salary 
FROM
	Employee,
	Department 
WHERE
	Employee.DepartmentId = Department.Id 
	AND ( Employee.DepartmentId, Salary ) 
    IN (SELECT DepartmentId, max( Salary ) 
        FROM Employee 
        GROUP BY DepartmentId )

思路 - join on结合

在思路一的基础上
通过join on进行融合

这种执行比较比较块

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
	)
;