Employee 表包含所有員工資訊，每個員工有其對應的 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 表包含公司所有部門的資訊。

+----+----------+
| Id | Name     |
+----+----------+
| 1  | IT       |
| 2  | Sales    |
+----+----------+

編寫一個 SQL 查詢，找出每個部門工資最高的員工。例如，根據上述給定的表格，Max 在 IT 部門有最高工資，Henry 在 Sales 部門有最高工資。

+------------+----------+--------+
| Department | Employee | Salary |
+------------+----------+--------+
| IT         | Max      | 90000  |
| Sales      | Henry    | 80000  |
+------------+----------+--------+

1、

select 
    Department.Name as Department, 
    Employee.Name as Employee, 
    Salary
from 
    Employee, Department
where 
    Employee.DepartmentId = Department.Id
and Salary = (
    select Max(Salary) 
from 
    Employee 
where 
    Employee.DepartmentId = Department.Id
)

執行用時: 1064 ms

2、

SELECT
    d.name AS 'Department',
    e.name AS 'Employee',
    Salary
FROM
    Employee e
JOIN
    Department d 
ON 
    e.DepartmentId = d.Id
WHERE(
    e.DepartmentId , Salary) 
IN(   
    SELECT
        DepartmentId, 
        MAX(Salary)
    FROM
        Employee
    GROUP BY 
        DepartmentId
  )

執行用時：297ms