Mysql多表查询，获取部门最高工资的sql语句方案_navicat premium 15每个部门的最高工资怎么算

建表的SQL的语句

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)); 
insert into Employee (Id, Name, Salary, DepartmentId) values ('1', 'Joe', '70000', '1'); 
insert into Employee (Id, Name, Salary, DepartmentId) values ('2', 'Henry', '80000', '2'); 
insert into Employee (Id, Name, Salary, DepartmentId) values ('3', 'Sam', '60000', '2'); 
insert into Employee (Id, Name, Salary, DepartmentId) values ('4', 'Max', '90000', '1'); 
insert into Department (Id, Name) values ('1', 'IT'); 
insert into Department (Id, Name) values ('2', 'Sales');


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分享一下group by在以下的多表查询中不可用,先给出错误的SQL语句,后面再给出正确语句和分析.

员工表employee,包括Id,Name,Salary,和DepartmentID,DepartmentID与部门id对应

部门表Department

需求 : 编写SQL语句查询每个部门中薪资最高的员工。每个部门只显示最高的工资的员工信息

错误的思路参考：
1、求各个部门最高工资

select max(salary) from employee group by DepartmentId

2、将最高工资表作虚拟表,查询出工资与最高工资相同的员工

select Department.name Department, employee.name Employee ,salary from employee join department on employee.DepartmentId = department.id where salary in (select max(salary) from employee group by DepartmentId)

貌似得到了想要的结果,但是出现以下情况时,得到的结果就是错误的:

当我改动joe的工资为80000时,得到的结果出乎意料:
为什么会出现这种情况?

where salary in
(select max(salary)
from employee
group by DepartmentId)

这里筛选条件相当是salary in (80000,90000)
下面给出正确的方式

select d.name Department,e1.name Employee,e1.salary Salary from employee e1 left join employee e2 on e1.salary < e2.salary and e1.DepartmentId=e2.DepartmentId join Department d on e1.DepartmentId = d.id where e2.id is null

一段一段看

select
*
from
employee e1 left join employee e2
on e1.salary < e2.salary and e1.DepartmentId=e2.DepartmentId

自连接,连接的条件是e1工资比e2工资低,部门号相同,这是当同部门中e1中没有比自己工资高的,自动补null,也就是上图中的情况

此时连接部门表,再筛选出e2的id是null的行就是所要求的