LeetCode - Department Highest Salary

题目大概的意思是选出每个Department里工资最高的人的信息并组成相应的表信息

有几个值得注意的地方：1）使用group by语句时，前面的select语句后面的内容只能有两种情况一种是group by后面的属性，另一种是聚集函数。

2）在选取最大Salary时必须使用e1.Salary=e2.Salary and e1.DepartmentId=e2.DepartmentId两个条件，要不然会有重复。

基于这些考虑可以使用派生表查询来找出最大Salary，然后与Department表做自然连接。（最后的升序还是降序无所谓）

select dep.Name as Department, pans.Name as Employee,
pans.Salary as Salary
from Department dep, (
select e1.* from
Employee e1, (select DepartmentId, max(Salary) as Salary
from Employee group by DepartmentId) e2
where e1.Salary=e2.Salary and e1.DepartmentId=e2.DepartmentId
) pans
where dep.Id=pans.DepartmentId
order by pans.Salary desc;