LeetCode：Department Top Three Salaries -各部门工资最高的三人

一、题目名称

Department Top Three Salaries（各部门工资最高的三我的）

二、题目地址

https://leetcode.com/problems/department-top-three-salaries/

三、题目内容

表Employee保存了全部雇员的数据，每名雇员都有一个Id，和一个部门Id（DepartmentId）

+----+-------+--------+--------------+
| Id | Name  | Salary | DepartmentId |
+----+-------+--------+--------------+
| 1  | Joe   | 70000  | 1            |
| 2  | Henry | 80000  | 2            |
| 3  | Sam   | 60000  | 2            |
| 4  | Max   | 90000  | 1            |
| 5  | Janet | 69000  | 1            |
| 6  | Randy | 85000  | 1            |
+----+-------+--------+--------------+

表Department保存了每一个部门的Id和名称

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

现须要写一个SQL，找出每一个部门工资最高的三名雇员。例如以上面的两张表的数据，查询出的结果集应该是这样的：

+------------+----------+--------+
| Department | Employee | Salary |
+------------+----------+--------+
| IT         | Max      | 90000  |
| IT         | Randy    | 85000  |
| IT         | Joe      | 70000  |
| Sales      | Henry    | 80000  |
| Sales      | Sam      | 60000  |
+------------+----------+--------+

四、初始化数据库脚本

在MySQL数据库中创建一个名为LEETCODE的数据库，用MySQL命令行中的source命令执行下面脚本：

-- 执行脚本前必须创建名为LEETCODE的DATABASE
USE LEETCODE;
    
DROP TABLE IF EXISTS Employee;
CREATE TABLE Employee (
  Id INT NOT NULL PRIMARY KEY,
  Name VARCHAR(10),
  Salary INT,
  DepartmentId INT
);
 
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (1, 'JID', 70000, 1);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (2, 'KAT', 80000, 2);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (3, 'FCK', 60000, 2);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (4, 'DIR', 90000, 1);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (5, 'CAT', 69000, 1);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (6, 'DOT', 85000, 1);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (7, 'GOS', 25000, 1);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (8, 'OPA', 90000, 2);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (9, 'OPB', 30000, 2);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (10, 'DIK', 30000, 3);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (11, 'POD', 30000, 3);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (12, 'SUV', 30000, 3);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (13, 'ABC', 30000, 3);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (15, 'WKK', 75000, 4);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (16, 'JYY', 60000, 4);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (17, 'XWY', 60000, 4);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (18, 'KMT', 55000, 4);
INSERT INTO Employee (Id, Name, Salary, DepartmentId) VALUES (14, 'LXK', 90000, 999);

DROP TABLE IF EXISTS Department;
CREATE TABLE Department (
  Id INT NOT NULL PRIMARY KEY,
  Name VARCHAR(10)
);
 
INSERT INTO Department (Id, Name) VALUES (1, 'DPRT1');
INSERT INTO Department (Id, Name) VALUES (2, 'DPRT2');
INSERT INTO Department (Id, Name) VALUES (3, 'DPRT3');
INSERT INTO Department (Id, Name) VALUES (4, 'DPRT4');

五、解题SQL

先写一个SQL，查询出每一个有部门的人的部门和工资

SELECT D.NAME Dep, E.NAME Emp, E.Salary Sal
FROM   Employee E
LEFT   JOIN Department D ON E.DepartmentId = D.Id
WHERE  D.NAME IS NOT NULL

使用 MySQL Query Browser 工具查询结果以下：

根据这个SQL，能够继续写出一个查询出每一个部门工资处于前三名位置的员工列表：

SELECT A.Dep Department, A.Emp Employee, A.Sal Salary
FROM   (SELECT D.NAME Dep, E.NAME Emp, E.Salary Sal
        FROM   Employee E
        LEFT   JOIN Department D ON E.DepartmentId = D.Id
        WHERE  D.NAME IS NOT NULL) A
WHERE  (SELECT COUNT(*)
        FROM   (SELECT D.NAME Dep, E.NAME Emp, E.Salary Sal
                FROM   Employee E
                LEFT   JOIN Department D ON E.DepartmentId = D.Id
                WHERE  D.NAME IS NOT NULL) B
        WHERE  A.Dep = B.Dep AND
               A.Sal < B.Sal) < 3
ORDER  BY A.Dep ASC, A.Sal DESC

这个SQL的查询结果集以下：

能够看到，部门4（DPRT4）有四我的，一个高工资（WKK，75000），一个低工资（KMT，55000），其余两我的工资（JYY，XWY，60000）是同样的。按照上面的SQL，会查出该部门工资最高的三我的是WKK、JYY、XWY，我认为这是一种合理的结果，即：

查出每一个部门工资排名最靠前的三我的，若有多于三人工资并列第一则将这些人都查出，若有多于两人工资并列第二则查出排名第一的人和这些并列第二的人，若有多于一人工资并列第三则查出排名第1、二名的人和全部并列第三名的人。

但这种相似计算长跑比赛前三名的算法却并非本题的“正确答案”，本题实际上要咱们找的并非要你求出每一个部门工资最高的三我的，而是要：

先求出每一个部门第三高的工资，再求出该部门工资数不小于第三高工资的全部雇员。

虽然我认为这是不合理的答案，不过仍是写了一个“正确”的SQL以下：

SELECT A.Dep Department, A.Emp Employee, A.Sal Salary
FROM   (SELECT D.NAME Dep, E.NAME Emp, E.Salary Sal
        FROM   Employee E
        LEFT   JOIN Department D ON E.DepartmentId = D.Id
        WHERE  D.NAME IS NOT NULL) A
WHERE  (SELECT COUNT(DISTINCT Sal)
        FROM   (SELECT D.NAME Dep, E.NAME Emp, E.Salary Sal
                FROM   Employee E
                LEFT   JOIN Department D ON E.DepartmentId = D.Id
                WHERE  D.NAME IS NOT NULL) B
        WHERE  A.Dep = B.Dep AND
               A.Sal < B.Sal) < 3
ORDER  BY A.Dep ASC, A.Sal DESC

查询出的结果集以下：

另外一种更简单的SQL写法以下，查询结果集与上同：

SELECT D.NAME Department, E.NAME Employee, E.Salary Salary
FROM   Department D, Employee E
WHERE  E.DepartmentId = D.Id AND
       (SELECT COUNT(DISTINCT Salary)
        FROM   Employee
        WHERE  DepartmentId = D.Id AND
               Salary > E.Salary) < 3

END