184. Department Highest Salary
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 |
+------------+----------+--------+
解答一：
SELECT D.Name AS Department ,E.Name AS Employee ,E.Salary
from
Employee E,
Department D
WHERE E.DepartmentId = D.id
AND (E.DepartmentId,E.Salary) in
(SELECT DepartmentId,max(Salary) as max FROM Employee GROUP BY DepartmentId)
解题思路是利用WHERE连接子句来构造筛选条件，子句汇中构造了一个新的表max，用到了IN操作符来选择DepartmentId 和Salary，group by 用来根据DepartmentId分组以便分别
求每个系的最高薪水。
解答二：
SELECT D.Name AS Department ,E.Name AS Employee ,E.Salary
FROM
Employee E,
(SELECT DepartmentId,max(Salary) as max FROM Employee GROUP BY DepartmentId) T,
Department D
WHERE E.DepartmentId = T.DepartmentId
AND E.Salary = T.max
AND E.DepartmentId = D.id
和一稍有不同，直接从三个表中选取，将新构造的表放到了FROM子句里边，其他直接以别名取代，以and连接三个条件。