LeetCode(184)-部门工资最高的员工

题目：

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 |
±-----------±---------±-------+

题解：

1是先通过分组找到每组薪资最高的，然后找到三个表共同的项

//1
select
d.name Department,
e.name Employee,
e.Salary
from
Employee e,
Department d,
(select DepartmentId ,max(Salary) Salary from Employee group by DepartmentId) m
where e.DepartmentId=d.Id
and e.DepartmentId=m.DepartmentId
and e.Salary=m.Salary;
//2 和上面一样先通过分组找每组工资最多的，通过将两张表并起来的方法找到。
select
d.Name Department,
e.Name Employee,
e.Salary
from
Employee e inner join
Department d
on e.DepartmentId=d.Id
where
(e.DepartmentId,e.Salary) in
(select
DepartmentId,max(Salary) as Salary
from Employee
group by DepartmentId
);