LeetCode：Department Highest Salary - 部門內最高工資

一、題目名稱

Department Highest Salary（部門內最高工資）

二、題目地址

https://leetcode.com/problems/department-highest-salary/

三、題目內容

表Employee包括四列：Id、Name、Salary、DepartmentId

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

表Department表中包括了公司的全部部門

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

現要求寫一個SQL，求出某一部門內領取最高工資的員工。

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

四、初始化數據庫腳本

在MySQL數據庫中創建一個名爲LEETCODE的數據庫，用MySQL命令行中的source命令執行下面腳本：

-- 執行腳本前必須創建名爲LEETCODE的DATABASE
USE LEETCODE;
 
DROP TABLE IF EXISTS Employee;
CREATE TABLE Employee (
  Id INT NOT NULL PRIMARY KEY,
  Name VARCHAR(50),
  Salary INT,
  DepartmentId INT
);

INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (1, 'Joe', 70000, 1);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (2, 'Henry', 80000, 2);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (3, 'Sam', 60000, 2);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (4, 'Max', 90000, 1);

DROP TABLE IF EXISTS Department;
CREATE TABLE Department (
  Id INT NOT NULL PRIMARY KEY,
  Name VARCHAR(50)
);

INSERT INTO Department (Id, Name) VALUES (1, 'IT');
INSERT INTO Department (Id, Name) VALUES (2, 'Sales');

五、解題SQL1

一個思路是，先求出各個部門的最高工資，再考察Employee表中，工資數等於所在部門的最高工資的員工，即爲所求。

SELECT D.Name AS Department, E.Name AS Employee, MAXS_D.MAXS AS Salary
FROM Employee AS E, Department AS D, (
  SELECT MAX(Salary) AS MAXS, DepartmentId
  FROM Employee
  GROUP BY DepartmentId) AS MAXS_D
WHERE E.Salary = MAXS_D.MAXS AND E.DepartmentId = D.Id AND 
  E.DepartmentId = MAXS_D.DepartmentId;

六、解題SQL2

另外一個思路是，在同一部門中，工資大於等於該部門全部員工的員工，即爲所求。

SELECT D.Name AS Department, E.Name AS Employee, E.Salary AS Salary
FROM Employee AS E, Department AS D
WHERE E.DepartmentId = D.Id AND Salary >= ALL(
  SELECT Salary 
  FROM Employee E_TMP
  WHERE E_TMP.DepartmentId = E.DepartmentId);

END