[LeetCode-DATABASE] Department Highest Salary

题目：

The Employee table holds all employees. Every employee has an Id, a salary, and there is also a column for the department Id.

+----+-------+--------+--------------+
| Id | Name  | Salary | DepartmentId |
+----+-------+--------+--------------+
| 1  | Joe   | 70000  | 1            |
| 2  | Henry | 80000  | 2            |
| 3  | Sam   | 60000  | 2            |
| 4  | Max   | 90000  | 1            |
+----+-------+--------+--------------+

The Department table holds all departments of the company.

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

Write a SQL query to find employees who have the highest salary in each of the departments. For the above tables, Max has the highest salary in the IT department and Henry has the highest salary in the Sales department.

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

题意：找到每个部门的最大工资的员工

思路：首先要明确，最大工资是唯一的，但是最大工资员工并不一定唯一，因此，要先内连接找到每个部门最大的工资，为了后续的方便，直接SELECT出对应的ID和NAME，这样只需要在和Employee做一次内连接就可以了。

SQL：

SELECT t.Name, e.Name, e.Salary
FROM Employee e,
    (
     SELECT e.DepartmentId AS Id, d.Name, MAX(e.Salary) AS MaxSalary
       FROM Employee e, Department d 
       WHERE e.DepartmentId = d.Id 
       GROUP BY d.Id
    ) t
WHERE e.Salary = t.MaxSalary AND e.DepartmentId = t.Id;

PS：前几天LeetCode的OJ有问题，总是Internal Error T.T