MySQL部门工资最高的员工

淡淡的烟草味﹌ 2023-10-06 18:50 12阅读 0赞

#### MySQL部门工资最高的员工 ####

*  SQL架构
 *  题目描述
 *  题解
 *   *  方法：使用 JOIN 和 IN 语句
     *  方法2
     *  方法： GROUP BY 语句
 *  知识点
 *   *  exists 和 in
     *   *  1.原理
         *  2.分析
         *  3.总结
         *  4.效率
         *  5.举例说明

## SQL架构 ##

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));
    
    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');
    
    insert into Department (Id, Name) values ('1', 'IT');
    insert into Department (Id, Name) values ('2', 'Sales');

## 题目描述 ##

Employee 表包含所有员工信息，每个员工有其对应的 Id, salary 和 department Id。

+----+-------+--------+--------------+
    | Id | Name  | Salary | DepartmentId |
    +----+-------+--------+--------------+
    | 1  | Joe   | 70000  | 1            |
    | 2  | Jim   | 90000  | 1            |
    | 3  | Henry | 80000  | 2            |
    | 4  | Sam   | 60000  | 2            |
    | 5  | Max   | 90000  | 1            |
    +----+-------+--------+--------------+

Department 表包含公司所有部门的信息。

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

编写一个 SQL 查询，找出每个部门工资最高的员工。对于上述表，您的 SQL 查询应返回以下行（行的顺序无关紧要）。

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

解释：

Max 和 Jim 在 IT 部门的工资都是最高的，Henry 在销售部的工资最高。

## 题解 ##

### 方法：使用 JOIN 和 IN 语句 ###

算法

因为 Employee 表包含 Salary 和 DepartmentId 字段，我们可以以此在部门内查询最高工资。

SELECT
        DepartmentId, MAX(Salary)
    FROM
        Employee
    GROUP BY DepartmentId;

> 注意：有可能有多个员工同时拥有最高工资，所以最好在这个查询中不包含雇员名字的信息。

+--------------+-------------+
    | DepartmentId | MAX(Salary) |
    +--------------+-------------+
    |            1 |       90000 |
    |            2 |       80000 |
    +--------------+-------------+
    2 rows in set (0.00 sec)

然后，我们可以把表 Employee 和 Department 连接，再在这张临时表里用 IN 语句查询部门名字和工资的关系。

SELECT
        Department.name AS 'Department',
        Employee.name AS 'Employee',
        Salary
    FROM
        Employee
            JOIN
        Department ON Employee.DepartmentId = Department.Id
    WHERE
        (Employee.DepartmentId , Salary) IN
        (   SELECT
                DepartmentId, MAX(Salary)
            FROM
                Employee
            GROUP BY DepartmentId
    	);

+------------+----------+--------+
    | Department | Employee | Salary |
    +------------+----------+--------+
    | IT         | Jim      |  90000 |
    | Sales      | Henry    |  80000 |
    | IT         | Max      |  90000 |
    +------------+----------+--------+
    3 rows in set (0.00 sec)

### 方法2 ###

select bb.Name as Department, aa.Employee, aa.Salary
    from (
    select 
           a.Id,
           a.Name as Employee,
           a.Salary,
           a.DepartmentId
      from Employee a
      join (
            select DepartmentId, max(Salary) as max_sal
              from Employee
             group by DepartmentId
           ) b on a.DepartmentId = b.DepartmentId and a.Salary = b.max_sal
    ) aa
    join Department bb on aa.DepartmentId = bb.Id;

### 方法： GROUP BY 语句 ###

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 );
    
    +------------+----------+--------+
    | Department | Employee | Salary |
    +------------+----------+--------+
    | IT         | Jim      |  90000 |
    | Sales      | Henry    |  80000 |
    | IT         | Max      |  90000 |
    +------------+----------+--------+
    3 rows in set (0.00 sec)
    
    SELECT D.Name AS Department,
           E1.Name AS Employee,
           E1.Salary
      FROM Employee AS E1
           INNER JOIN Department AS D
           ON E1.DepartmentId = D.Id
     WHERE NOT EXISTS (SELECT * 
                         FROM Employee AS E2
                        WHERE E1.DepartmentId = E2.DepartmentId
                          AND E1.Salary < E2.Salary);

## 知识点 ##

### exists 和 in ###

#### 1.原理 ####

通过使用 EXISTS，Oracle会首先检查主查询，然后运行子查询直到它找到第一个匹配项，  
这就节省了时间。

Oracle在执行IN子查询时，首先执行子查询，并将获得的结果列表存放在一个加了索引的临时表中。

在执行子查询之前，系统先将主查询挂起，待子查询执行完毕，存放在临时表中以后再执行主查询。

这也就是使用EXISTS比使用IN通常查询速度快的原因。

#### 2.分析 ####

in 是把外表和内表作hash 连接，而exists是对外表作loop循环。

每次loop循环再对内表进行查询not exists:做NLL，对子查询先查，有个虚表，有确定值，所以就算子查询有NULL最终也有值返回not in:做hash，对子查询表建立内存数组，用外表匹配，那子查询要是NULL那外表没的匹配最终无值返回。

一直以来认为exists比in效率高的说法是不准确的。

> 但是，如果查询的两个表大小相当，那么用in和exists差别不大。

#### 3.总结 ####

外表大，用IN；内表大，用EXISTS。

#### 4.效率 ####

- select * from T1 where exists(select 1 from T2 where T1.a=T2.a) ;
        T1数据量小而T2数据量非常大时，T1<<T2 时，1) 的查询效率高。
    - select * from T1 where T1.a in (select T2.a from T2);
        T1数据量非常大而T2数据量小时，T1>>T2 时，2) 的查询效率高。

#### 5.举例说明 ####

例如：表A（小表），表B（大表） 
     1： select * from A where cc in (select cc from B) 
         效率低，用到了A表上cc列的索引； 
        select * from A where exists(select cc from B where cc=A.cc) 
        效率高，用到了B表上cc列的索引。
     
     2. select * from B where cc in (select cc from A) 
        效率高，用到了B表上cc列的索引； 
        select * from B where exists(select cc from A where cc=B.cc) 
        效率低，用到了A表上cc列的索引。