sql 184. 部门工资最高的员工

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

select 
    d.Name as Department,
    e.Name as Employee,
    e.Salary 
from 
    Employee e,Department d 
where
    e.DepartmentId=d.id 
    and
    (e.Salary , e.DepartmentId) in (select max(Salary),DepartmentId from Employee group by DepartmentId);

此次学到的用法为把查询到的结果当做另一次查询的条件

另一个答案为

select 
t.Name as Department,
e.Name as Employee, 
e.Salary as Salary
from(
    select DepartmentId, 
        d.name as Name ,
        max(Salary) as Salary from Employee  e
    join Department d on d.Id = e.DepartmentId 
    group by DepartmentId) as t
join Employee e
on t.DepartmentId = e.DepartmentId and e.Salary = t.Salary

由此答案可得出select后的结果可给别的select当源，但个数必须匹配