LeetCode.184. 部門工資最高的員工

題目

SQL 架構

Create table If Not Exists Employee (Id int, Name varchar(255), Salary int, DepartmentId int)
Create table If Not Exists Department (Id int, Name varchar(255))
Truncate table Employee
insert into Employee (Id, Name, Salary, DepartmentId) values ('1', 'Joe', '70000', '1')
insert into Employee (Id, Name, Salary, DepartmentId) values ('2', 'Jim', '90000', '1')
insert into Employee (Id, Name, Salary, DepartmentId) values ('3', 'Henry', '80000', '2')
insert into Employee (Id, Name, Salary, DepartmentId) values ('4', 'Sam', '60000', '2')
insert into Employee (Id, Name, Salary, DepartmentId) values ('5', 'Max', '90000', '1')
Truncate table Department
insert into Department (Id, Name) values ('1', 'IT')
insert into Department (Id, Name) values ('2', 'Sales')

Employee 表包含所有員工信息，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信息。

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

題解

回憶一下 GROUP BY 語句

GROUP BY 字段名   # 根據字段進行分組

先對 DepartmentId 字段分組查詢最大值，得到不同 DepartmentId 下的最大值

SELECT DepartmentId, max( Salary ) 
FROM Employee 
GROUP BY DepartmentId 

再根據 DepartmentId 字段連接 Department 表，根據 Salary 和 DepartmentId 查找 Department.Name 字段

SELECT
	Department.NAME AS Department,
	Employee.NAME AS Employee,
	Salary 
FROM
	Employee,
	Department 
WHERE
	Employee.DepartmentId = Department.Id 
	AND ( Employee.DepartmentId, Salary ) 
    IN (SELECT DepartmentId, max( Salary ) 
        FROM Employee 
        GROUP BY DepartmentId )