6、子查询、having、(not) exists、表中增加一列并补充数据、删除表中对应的数据-mysql

╰+攻爆jí腚メ 2024-03-31 08:54 18阅读 0赞

#### 目录 ####

*  子查询
 *   *  一、需求分析与问题解决
     *   *   *  1、实际问题
             *  2、子查询的基本使用
             *  3、子查询的分类
     *  二、单行子查询
     *   *   *  1、单行比较操作符
             *  2、 代码示例
             *  3、HAVING 中的子查询
             *  4、CASE中的子查询
             *  5、 子查询中的空值问题
             *  6、 非法使用子查询
     *  三、多行子查询
     *   *   *  1、多行比较操作符
             *  2、代码示例
             *  3、空值问题
     *  四、 相关子查询
     *   *   *  1、相关子查询执行流程
             *  2、代码示例
             *  3、EXISTS 与 NOT EXISTS关键字
             *  4、相关更新
             *  5、相关删除
     *  五、抛一个思考题
     *  六、子查询的课后练习

## 子查询 ##

—————————————————————————————————————————————  
`子查询`指**一个查询语句嵌套在另一个查询语句内部的查询**，这个特性从MySQL 4.1开始引入。

`SQL` 中子查询的使用大大增强了 `SELECT` 查询的能力，因为很多时候查询需要从 **结果集** 中获取数据，或者需要从 **同一个表** 中先计算得出一个**数据结果**，然后与这个数据结果（可能是某个标量，也可能是某个集合）进行比较。

### 一、需求分析与问题解决 ###

##### 1、实际问题 #####

![在这里插入图片描述][d3d0b0d1f0d249c2be169bc0472809d6.png]  
**现有解决方式：**

#方式一：
    SELECT salary
    FROM employees
    WHERE last_name = 'Abel';
    
    SELECT last_name,salary
    FROM employees
    WHERE salary > 11000;
    
    #方式二：自连接
    SELECT e2.last_name,e2.salary
    FROM employees e1,employees e2
    WHERE e1.last_name = 'Abel'
    AND e1.`salary` < e2.`salary`

#方式三：子查询
    SELECT last_name,salary
    FROM employees
    WHERE salary > (
    		SELECT salary
    		FROM employees
    		WHERE last_name = 'Abel'
    		);

--------------------

##### 2、子查询的基本使用 #####

*  子查询的基本语法结构：  
    ![在这里插入图片描述][1a4d6bd551b34fa5aa2133c254a503ba.png]
 *  子查询（内查询）在主查询之前一次执行完成。
 *  子查询的结果被主查询（外查询）使用 。
 *  **注意事项**
    
     *  子查询要包含在括号内
     *  将子查询放在比较条件的右侧
     *  单行操作符对应单行子查询，多行操作符对应多行子查询

--------------------

##### 3、子查询的分类 #####

*  **分类方式1：**
    
    我们按 **内查询** 的结果返回`一条`还是`多条记录`，将子查询分为**单行子查询**、**多行子查询**。
    
     *  **单行子查询** 也可以包含一行多个列  
        ![在这里插入图片描述][f4f9e7fee7f442acbda237c0e8a56e3f.png]
     *  **多行子查询**  
        ![在这里插入图片描述][ee011deae8a94523aa777a14fc667082.png]
 *  **分类方式2：**
    
    我们按 **内查询是否被执行多次**，将子查询划分为 **相关(或关联)子查询** 和 **不相关(或非关联)子查询**。
    
     *  子查询从数据表中查询了数据结果，如果这个`数据结果只执行一次`，然后这个数据结果作为主查询的条件进行执行，那么这样的子查询叫做**不相关子查询**。
     *  同样，如果子查询需要`执行多次`，即采用循环的方式，先从外部查询开始，每次都传入子查询进行查询，然后再将结果反馈给外部，这种嵌套的执行方式就称为**相关子查询**。

--------------------

### 二、单行子查询 ###

##### 1、单行比较操作符 #####

<table> 
 <thead> 
  <tr> 
   <th>操作符</th> 
   <th>含义</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>=</td> 
   <td>equal to</td> 
  </tr> 
  <tr> 
   <td>&gt;</td> 
   <td>greater than</td> 
  </tr> 
  <tr> 
   <td>&gt;=</td> 
   <td>greater than or equal to</td> 
  </tr> 
  <tr> 
   <td>&lt;</td> 
   <td>less than</td> 
  </tr> 
  <tr> 
   <td>&lt;=</td> 
   <td>less than or equal to</td> 
  </tr> 
  <tr> 
   <td>&lt;&gt;</td> 
   <td>not equal to</td> 
  </tr> 
 </tbody> 
</table>

--------------------

##### 2、 代码示例 #####

*  **题目：查询工资大于149号员工工资的员工的信息**

![在这里插入图片描述][0363ff56cb07486cb346d9698b4ebb47.png]

*  题目：返回job\_id与141号员工相同，salary比143号员工多的员工姓名，job\_id和工资
    
        SELECT last_name, job_id, salary
        FROM   employees
        WHERE  job_id =  
                        (SELECT job_id
                         FROM   employees
                         WHERE  employee_id = 141)
        AND    salary >
                        (SELECT salary
                         FROM   employees
                         WHERE  employee_id = 143);
 *  题目：返回公司工资最少的员工的last\_name,job\_id和salary
    
        SELECT last_name, job_id, salary
        FROM   employees
        WHERE  salary = 
                        (SELECT MIN(salary)
                         FROM   employees);
 *  题目：查询与141号或174号员工的manager\_id和department\_id相同的其他员工的employee\_id，manager\_id，department\_id
    
     *  实现方式1：不成对比较
        
            SELECT  employee_id, manager_id, department_id
            FROM    employees
            WHERE   manager_id IN
            		  (SELECT  manager_id
                               FROM    employees
                               WHERE   employee_id IN (174,141))
            AND     department_id IN 
            		  (SELECT  department_id
                               FROM    employees
                               WHERE   employee_id IN (174,141))
            AND	employee_id NOT IN(174,141);
     *  实现方式2：成对比较
        
            SELECT	employee_id, manager_id, department_id
            FROM	employees
            WHERE  (manager_id, department_id) IN
                                  (SELECT manager_id, department_id
                                   FROM   employees
                                   WHERE  employee_id IN (141,174))
            AND	employee_id NOT IN (141,174);

--------------------

##### 3、HAVING 中的子查询 #####

*  首先执行子查询。
 *  向主查询中的 `HAVING` 子句返回结果。

**题目：查询最低工资大于50号部门最低工资的部门id和其最低工资**

SELECT   department_id, MIN(salary)
    FROM     employees
    GROUP BY department_id
    HAVING   MIN(salary) >
                           (SELECT MIN(salary)
                            FROM   employees
                            WHERE  department_id = 50);

--------------------

##### 4、CASE中的子查询 #####

在CASE表达式中使用单列子查询：

题目：显式员工的employee\_id,last\_name和location。其中，若员工department\_id与location\_id为1800的department\_id相同，则location为’Canada’，其余则为’USA’。

SELECT employee_id, last_name,
           (CASE department_id
            WHEN
                 (SELECT department_id FROM departments
    	      WHERE location_id = 1800)           
            THEN 'Canada' ELSE 'USA' END) location
    FROM   employees;

--------------------

##### 5、 子查询中的空值问题 #####

SELECT last_name, job_id
    FROM   employees
    WHERE  job_id =
                    (SELECT job_id
                     FROM   employees
                     WHERE  last_name = 'Haas');
    # 语句解释：
    #	因为数据中没有 last_name = 'Haas' 的数据，所以 job_id 为 null, 外层用 job_id = null去查询所以显示为null

![在这里插入图片描述][4f17f0e8a0fb492da6f69d4d40990f6a.png]

> 子查询不返回任何行

--------------------

##### 6、 非法使用子查询 #####

SELECT employee_id, last_name
    FROM   employees
    WHERE  salary =
                    (SELECT   MIN(salary)
                     FROM     employees
                     GROUP BY department_id);
    # 语句解释：
    #	因为子查询语句中 select min(salary) from employees group by department_id 查询出了多条数据
    #	那么 salary 不知道使用哪个去等
    # 怎么改？用多行子查询关键字 in

![在这里插入图片描述][e668313d335247a4bcfb8f8825ed77da.png]

> 多行子查询使用单行比较符

### 三、多行子查询 ###

*  也称为集合比较子查询
 *  内查询返回多行数据
 *  使用多行比较操作符

##### 1、多行比较操作符 #####

<table> 
 <thead> 
  <tr> 
   <th>操作符</th> 
   <th>含义</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>IN</td> 
   <td>等于列表中的<code>任意一个</code></td> 
  </tr> 
  <tr> 
   <td>ANY</td> 
   <td>需要和单行比较操作符一起使用，和子查询返回的<code>某一个</code>值比较</td> 
  </tr> 
  <tr> 
   <td>ALL</td> 
   <td>需要和单行比较操作符一起使用，和子查询返回的<code>所有值</code>比较</td> 
  </tr> 
  <tr> 
   <td>SOME</td> 
   <td>实际上是ANY的别名，作用相同，一般常使用ANY</td> 
  </tr> 
 </tbody> 
</table>

> 体会 ANY 和 ALL 的区别 @TODO

--------------------

##### 2、代码示例 #####

题目：返回其它 job_id 中比 job_id 为 IT_PROG 部门任一工资低的员工的员工号、姓名、job_id 以及salary

SELECT employee_id, last_name, job_id, salary 
    FROM employees
    WHERE salary < ANY 
    	(SELECT salary FROM employees WHERE job_id = 'IT_PROG')
    AND job_id <> 'IT_PROG';

![在这里插入图片描述][c5b17529058e41f484161fa0f093abc1.png]

题目：返回其它job_id中比job_id为‘IT_PROG’部门所有工资都低的员工的员工号、姓名、job_id以及salary

![在这里插入图片描述][fdf919b3c75a412386662c544fa44640.png]

题目：查询平均工资最低的部门id

#方式1：
    SELECT department_id
    FROM employees
    GROUP BY department_id
    HAVING AVG(salary) = (
    			SELECT MIN(avg_sal)
    			FROM (
    				SELECT AVG(salary) avg_sal
    				FROM employees
    				GROUP BY department_id
    				) dept_avg_sal
    			)

#方式2：
    SELECT department_id
    FROM employees
    GROUP BY department_id
    HAVING AVG(salary) <= ALL (
    				SELECT AVG(salary) avg_sal
    				FROM employees
    				GROUP BY department_id
    )

--------------------

##### 3、空值问题 #####

SELECT last_name
    FROM employees
    WHERE employee_id NOT IN (
    			SELECT manager_id
    			FROM employees
    			);

![在这里插入图片描述][94e03b9270c24a4eaea18cc849bcf503.png]

--------------------

### 四、 相关子查询 ###

##### 1、相关子查询执行流程 #####

如果子查询的执行依赖于外部查询，通常情况下都是因为子查询中的表用到了外部的表，并进行了条件关联，因此每执行一次外部查询，子查询都要重新计算一次，这样的子查询就称之为关联子查询。

相关子查询按照一行接一行的顺序执行，主查询的每一行都执行一次子查询。  
![在这里插入图片描述][e89710d1f9bd48b7b8a53524b74a7990.png]

> 说明：子查询中使用主查询中的列

--------------------

##### 2、代码示例 #####

题目：查询员工中工资大于本部门平均工资的员工的last_name,salary和其department_id

*  **方式一：相关子查询**  
    ![在这里插入图片描述][244f8e685f5e4c7799bce34f30639bf5.png]
 *  **方式二：在 FROM 中使用子查询**
    
        SELECT last_name,salary,e1.department_id
        FROM employees e1,(SELECT department_id,AVG(salary) dept_avg_sal FROM employees GROUP BY department_id) e2
        WHERE e1.`department_id` = e2.department_id
        AND e2.dept_avg_sal < e1.`salary`;

> from型的子查询：子查询是作为from的一部分，子查询要用()引起来，并且要给这个子查询取别名，  
> 把它当成一张“临时的虚拟的表”来使用。

*  在`ORDER BY` 中使用子查询：
    
        题目：查询员工的id,salary,按照department_name 排序
    
        SELECT employee_id,salary
        FROM employees e
        ORDER BY (
        	  SELECT department_name
        	  FROM departments d
        	  WHERE e.`department_id` = d.`department_id`
        	);
    
        题目：若employees表中employee_id与job_history表中employee_id相同的数目不小于2，
         	  输出这些相同id的员工的employee_id,last_name和其job_id
    
        SELECT e.employee_id, last_name,e.job_id
        FROM   employees e 
        WHERE  2 <= (SELECT COUNT(*)
                     FROM   job_history 
                     WHERE  employee_id = e.employee_id);

--------------------

##### 3、EXISTS 与 NOT EXISTS关键字 #####

*  关联子查询通常也会和 `EXISTS`操作符一起来使用，用来检查在子查询中是否存在满足条件的行。
 *  **如果在子查询中不存在满足条件的行：**
    
     *  条件返回 `FALSE`
     *  继续在子查询中查找
 *  **如果在子查询中存在满足条件的行：**
    
     *  不在子查询中继续查找
     *  条件返回 TRUE
 *  `NOT EXISTS`关键字表示如果不存在某种条件，则返回TRUE，否则返回FALSE。
    
        题目：查询公司管理者的employee_id，last_name，job_id，department_id信息
 *  **方式一：**
    
        SELECT employee_id, last_name, job_id, department_id
        FROM   employees e1
        WHERE  EXISTS ( SELECT *
                         FROM   employees e2
                         WHERE  e1.employee_id = e2.manager_id);
 *  **方式二：自连接**
    
        SELECT DISTINCT e1.employee_id, e1.last_name, e1.job_id, e1.department_id
        FROM   employees e1 JOIN employees e2
        WHERE e1.employee_id = e2.manager_id;
 *  **方式三：**
    
        SELECT employee_id,last_name,job_id,department_id
        FROM employees
        WHERE employee_id IN (
        		     SELECT DISTINCT manager_id
        		     FROM employees
        		     );
    
        题目：查询departments表中，不存在于employees表中的部门的department_id和department_name
    
        # 方式一：
        	select d.department_id, d.department_name
        		from employees e right join departments d
        		on e.department_id = d.department_id
        		where e.department_id is null;
        	# 方式二：
        	SELECT department_id, department_name
        	FROM departments d
        	WHERE NOT EXISTS (SELECT 'X'
        	                  FROM   employees
        	                  WHERE  department_id = d.department_id);

--------------------

##### 4、相关更新 #####

UPDATE table1 alias1
    SET    column = (SELECT expression
                     FROM   table2 alias2
                     WHERE  alias1.column = alias2.column);

使用相关子查询依据一个表中的数据更新另一个表的数据。

题目：在employees中增加一个department_name字段，数据为员工对应的部门名称

# 1）
    ALTER TABLE employees
    ADD(department_name VARCHAR2(14));
    
    # 2）
    UPDATE employees e
    SET department_name =  (SELECT department_name 
    	                       FROM   departments d
    	                       WHERE  e.department_id = d.department_id);

##### 5、相关删除 #####

DELETE FROM table1 alias1
     WHERE column operator (SELECT expression
                            FROM   table2 alias2
                            WHERE  alias1.column = alias2.column);

使用相关子查询依据一个表中的数据删除另一个表的数据。

题目：删除表employees中，其与emp_history表皆有的数据

DELETE FROM employees e
    WHERE employee_id in  
               (SELECT employee_id
                FROM   emp_history 
                WHERE  employee_id = e.employee_id);

--------------------

### 五、抛一个思考题 ###

*  **问题**：谁的工资比Abel的高？
 *  **解答：**
    
        #方式1：自连接
        SELECT e2.last_name,e2.salary
        FROM employees e1,employees e2
        WHERE e1.last_name = 'Abel'
        AND e1.`salary` < e2.`salary`
    
        #方式2：子查询
        SELECT last_name,salary
        FROM employees
        WHERE salary > (
        		SELECT salary
        		FROM employees
        		WHERE last_name = 'Abel'
        		);
 *  **问题**：以上两种方式有好坏之分吗？
 *  **解答**：自连接方式好！
    
    题目中可以使用子查询，也可以使用自连接。一般情况建议你使用自连接，因为在许多 DBMS 的处理过程中，对于自连接的处理速度要比子查询快得多。
    
    可以这样理解：子查询实际上是通过未知表进行查询后的条件判断，而自连接是通过已知的自身数据表进行条件判断，因此在大部分 DBMS 中都对自连接处理进行了优化。

--------------------

### 六、子查询的课后练习 ###

# 1、查询和 Zlotkey 相同部门的员工姓名和工资
    SELECT last_name, salary 
    FROM employees 
    WHERE department_id IN (SELECT department_id FROM employees WHERE last_name = 'Zlotkey')
    
    # 2、查询工资比公司平均工资高的员工的员工号，姓名和工资。
    SELECT employee_id, last_name,salary
    FROM employees
    WHERE salary > (SELECT AVG(salary) FROM employees)
    # 3、选择工资大于所有 JOB_ID = 'SA_MAN' 的员工的工资的员工的 last_name,job_id, salary
    SELECT last_name,job_id,salary
    FROM employees
    WHERE salary > ALL (SELECT salary FROM employees WHERE job_id = 'SA_MAN') 
    
    # 4、查询和姓名中包含字母 u 的员工在相同部门的员工的员工号和姓名
    SELECT employee_id,last_name
    FROM employees
    WHERE department_id IN (SELECT DISTINCT department_id FROM employees WHERE last_name LIKE '%u%')
    
    # 5、查询在部门的 location_id 为 1700 的部门工作的员工的员工号
    # 方式一：
    SELECT employee_id
    FROM employees e JOIN departments d ON e.department_id = d.department_id
    WHERE d.location_id = '1700'
    # 方式二：
    SELECT employee_id 
    FROM employees
    WHERE department_id IN (SELECT department_id FROM departments WHERE location_id = '1700')
    
    # 6、查询管理者是 King 的员工姓名和工资
    SELECT last_name,salary
    FROM employees
    WHERE manager_id IN (SELECT employee_id FROM employees WHERE last_name = 'King')
    
    # 7、查询工资最低的员工信息：last_name,salary
    SELECT last_name, salary
    FROM employees
    WHERE salary = (SELECT MIN(salary) FROM employees)
    
    # 8、查询平均工资最低的部门信息
    # 方式一：
    SELECT * FROM departments WHERE department_id = (SELECT department_id
    FROM employees
    GROUP BY department_id 
    HAVING AVG(salary) = (
    SELECT MIN(avg_salary)  
    FROM 
    (SELECT AVG(salary) avg_salary
    FROM employees
    GROUP BY department_id) vi_dept_sal))
    
    # 方式二
    SELECT * FROM departments WHERE department_id = (SELECT department_id
    FROM employees
    GROUP BY department_id 
    HAVING AVG(salary) <= ALL (
    SELECT AVG(salary) avg_salary
    FROM employees
    GROUP BY department_id) )
    
    # 方式三：
    SELECT * FROM departments WHERE department_id = (SELECT department_id
    FROM employees
    GROUP BY department_id 
    HAVING AVG(salary) = (SELECT AVG(salary) avg_salary
    FROM employees
    GROUP BY department_id
    ORDER BY avg_salary ASC
    LIMIT 1))
    
    # 方式四：
    SELECT d.* FROM departments d,(
    SELECT department_id, AVG(salary) avg_sal
    FROM employees
    GROUP BY department_id
    ORDER BY avg_sal ASC
    LIMIT 1) t_dept_avg_sal
    WHERE d.department_id = t_dept_avg_sal.department_id
    
    # 9、查询平均工资最低的部门信息和该部门的平均工资（相关子查询）
    # 方式一：
    SELECT d.* ,(SELECT AVG(salary) FROM employees WHERE department_id = d.department_id) avf_sal
    FROM departments d WHERE department_id = (SELECT department_id
    FROM employees
    GROUP BY department_id 
    HAVING AVG(salary) = (
    SELECT MIN(avg_salary)  
    FROM 
    (SELECT AVG(salary) avg_salary
    FROM employees
    GROUP BY department_id) vi_dept_sal))
    
    # 方式二
    SELECT * ,(SELECT AVG(salary) FROM employees WHERE department_id = d.department_id) avf_sal
    FROM departments d WHERE department_id = (SELECT department_id
    FROM employees
    GROUP BY department_id 
    HAVING AVG(salary) <= ALL (
    SELECT AVG(salary) avg_salary
    FROM employees
    GROUP BY department_id) )
    
    # 方式三：
    SELECT * ,(SELECT AVG(salary) FROM employees WHERE department_id = d.department_id) avf_sal
    FROM departments d WHERE department_id = (SELECT department_id
    FROM employees
    GROUP BY department_id 
    HAVING AVG(salary) = (SELECT AVG(salary) avg_salary
    FROM employees
    GROUP BY department_id
    ORDER BY avg_salary ASC
    LIMIT 1))
    
    # 方式四：
    SELECT d.* ,(SELECT AVG(salary) FROM employees WHERE department_id = d.department_id) avf_sal
    FROM departments d,(
    SELECT department_id, AVG(salary) avg_sal
    FROM employees
    GROUP BY department_id
    ORDER BY avg_sal ASC
    LIMIT 1) t_dept_avg_sal
    WHERE d.department_id = t_dept_avg_sal.department_id
    
    # 10、查询平均工资最高的 job 信息
    # 方式一：
    SELECT * FROM jobs WHERE job_id = (SELECT job_id
    FROM employees
    GROUP BY job_id 
    HAVING AVG(salary) = (
    SELECT MAX(avg_salary)  
    FROM 
    (SELECT AVG(salary) avg_salary
    FROM employees
    GROUP BY job_id) vi_dept_sal))
    
    # 方式二
    SELECT * FROM jobs WHERE job_id = (SELECT job_id
    FROM employees
    GROUP BY job_id 
    HAVING MAX(salary) >= ALL (
    SELECT AVG(salary) avg_salary
    FROM employees
    GROUP BY job_id) )
    
    # 方式三：
    SELECT * FROM jobs WHERE job_id = (SELECT job_id
    FROM employees
    GROUP BY job_id 
    HAVING AVG(salary) = (SELECT AVG(salary) avg_salary
    FROM employees
    GROUP BY job_id
    ORDER BY avg_salary DESC
    LIMIT 1))
    
    # 方式四：
    SELECT d.* FROM jobs d,(
    SELECT job_id, AVG(salary) avg_sal
    FROM employees
    GROUP BY job_id
    ORDER BY avg_sal DESC
    LIMIT 1) t_dept_avg_sal
    WHERE d.job_id = t_dept_avg_sal.job_id  
    
    # 11、查询 平均工资 高于 公司平均工资 的部门有哪些？
    
    SELECT department_id
    FROM employees
    WHERE department_id IS NOT NULL
    GROUP BY department_id
    HAVING AVG(salary) >= (
    			SELECT AVG(salary) FROM employees
    			)
     
    # 12、查询出公司中所有 manager 的详细信息
    # 方式一：自查询
    SELECT DISTINCT e1.employee_id, e1.last_name, e1.job_id, e1.department_id
    FROM   employees e1 JOIN employees e2
    WHERE e1.employee_id = e2.manager_id;
    
    # 方式二：子查询
    SELECT employee_id,last_name,job_id,department_id
    FROM employees
    WHERE employee_id IN (
    		     SELECT DISTINCT manager_id
    		     FROM employees
    		     
    		     );
    # 方式三：
    SELECT employee_id, last_name, job_id, department_id
    FROM   employees e1
    WHERE  EXISTS ( SELECT *
                     FROM   employees e2
                     WHERE  e2.manager_id = 
                            e1.employee_id);
    
    # 13、各个部门中 最高工资中 最低的 那个部门的 最低工资是多少？
    # 方式一：
    SELECT MIN(salary) 
    FROM employees 
    WHERE department_id = (
    SELECT department_id 
    FROM employees
    GROUP BY department_id
    HAVING MAX(salary) = (
    SELECT MIN(max_sal)
    FROM (
    SELECT MAX(salary) max_sal
    FROM employees
    GROUP BY department_id) t_dept_max_sal ))
    
    # 方式二：
    SELECT MIN(salary) 
    FROM employees 
    WHERE department_id = (
    SELECT department_id 
    FROM employees
    GROUP BY department_id
    HAVING MAX(salary) <= ALL (
    SELECT MAX(salary) max_sal
    FROM employees
    GROUP BY department_id ))
    
    # 方式三：
    SELECT MIN(salary) 
    FROM employees 
    WHERE department_id = (
    SELECT department_id 
    FROM employees
    GROUP BY department_id
    HAVING MAX(salary) = (
    SELECT MAX(salary) max_sal
    FROM employees
    GROUP BY department_id
    ORDER BY max_sal
    LIMIT 1))
    
    # 方式四：
    SELECT MIN(salary)
    FROM employees e, (SELECT department_id, MAX(salary) max_sal
    FROM employees
    GROUP BY department_id
    ORDER BY max_sal
    LIMIT 1) t_dept_max_sal
    WHERE t_dept_max_sal.department_id = e.department_id
    
    # 14、查询 平均工资 最高的部门的 manager 的详细信息：last_name, department_id, email, salary
    # 方式一：
    SELECT last_name,department_id,email,salary
    FROM employees 
    WHERE employee_id IN (
    SELECT DISTINCT manager_id
    FROM employees
    WHERE department_id = (
    SELECT department_id 
    FROM employees
    GROUP BY department_id
    HAVING AVG(salary) = (
    SELECT MAX(avg_sal)
    FROM (SELECT AVG(salary) avg_sal
    FROM employees
    GROUP BY department_id ) t_dept_avg_sal)))
    # 方式二：（与上面同思路）
    # 方式三：（与上面同思路）
    # 方式四：（与上面同思路）
    
    # 15、查询部门的部门号，其中不包含 job_id 是 "ST_CLERK" 的部门号
    # 方式一：
    SELECT department_id 
    FROM departments
    WHERE department_id NOT IN (SELECT DISTINCT department_id FROM employees WHERE job_id = 'ST_CLERK')
    
    # 方式二：
    SELECT department_id 
    FROM departments d
    WHERE NOT EXISTS (
    		SELECT * FROM employees e WHERE d.department_id = e.`department_id`
    		AND job_id = 'ST_CLERK'
    )
    # 16、选择所有没有管理者的员工的 last_name
    # 方式一：
    SELECT last_name 
    FROM employees
    WHERE manager_id IS NULL
    
    # 方式二：
    SELECT last_name
    FROM employees emp
    WHERE NOT EXISTS (
    		SELECT * FROM employees mgr 
    		WHERE emp.manager_id = mgr.employee_id
    )
    # 17、查询员工号、姓名、雇用时间、工资，其中员工的管理者为'De Haan'
    # 方式一：
    SELECT employee_id,last_name,hire_date,salary
    FROM employees
    WHERE manager_id IN (
    	SELECT employee_id FROM employees WHERE last_name = 'De Haan'
    )
    
    # 方式二：
    SELECT employee_id,last_name,hire_date,salary
    FROM employees emp
    WHERE EXISTS (
    	SELECT * FROM employees mgr
    	WHERE emp.manager_id = mgr.employee_id
    	AND mgr.last_name = 'De Haan'
    )
    # 18、查询各部门中工资比本部门平均工资高的员工的员工号，姓名和工资（相关查询）
    SELECT last_name,salary,e1.department_id
    FROM employees e1,(SELECT department_id,AVG(salary) dept_avg_sal FROM employees GROUP BY department_id) e2
    WHERE e1.`department_id` = e2.department_id
    AND e2.dept_avg_sal < e1.`salary`;
    
    
    # 19、查询每个部门下的部门人数大于 5 的部门名称（相关查询）
    SELECT department_name
    FROM departments d
    WHERE 5 < (
    	SELECT COUNT(*)
    	FROM employees e 
    	WHERE d.`department_id` = e.`department_id`
    )
    
    
    # 20、查询每个国家下的部门个数大于 2 的国家编号（相关查询）
    
    SELECT country_id 
    FROM locations l
    WHERE 2 < (
    	SELECT COUNT(*)
    	FROM departments d
    	WHERE l.location_id = d.`location_id`
    )

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