MySQL笔记：第09章_子查询

第09章_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。

1. 需求分析与问题解决

1.1 实际问题

请添加图片描述
现有解决方式：

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#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'
);
1.2 子查询的基本使用
  • 子查询的基本语法结构：

请添加图片描述

  • 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
1.3 子查询的分类
分类方式1：

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
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分类方式2：

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。

演示代码
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#1. 由一个具体的需求，引入子查询
#需求：谁的工资比Abel的高？
#方式1（效率低，交互次数多）：
SELECT salary
FROM employees
WHERE last_name = 'Abel';
/*输出：
+----------+
| salary   |
+----------+
| 11000.00 |
+----------+
*/
SELECT last_name,salary
FROM employees
WHERE salary > 11000;
/*部分输出：
+-----------+----------+
| last_name | salary   |
+-----------+----------+
| King      | 24000.00 |
| Kochhar   | 17000.00 |
| De Haan   | 17000.00 |
| Greenberg | 12000.00 |
| Russell   | 14000.00 |
| Partners  | 13500.00 |
*/

#方式2：自连接
SELECT e2.last_name,e2.salary
FROM employees e1,employees e2
WHERE e2.`salary` > e1.`salary` #多表的连接条件（非等值连接）
AND e1.last_name = 'Abel';

#方式3：子查询
SELECT last_name,salary
FROM employees
WHERE salary > (
		SELECT salary
		FROM employees
		WHERE last_name = 'Abel'
		);

#2. 称谓的规范：外查询（或主查询）、内查询（或子查询）
/*
- 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
*/

#不推荐：
SELECT last_name,salary
FROM employees
WHERE  (
	SELECT salary
	FROM employees
	WHERE last_name = 'Abel'
		) < salary;
	
/*
3. 子查询的分类
角度1：从内查询返回的结果的条目数
	单行子查询  vs  多行子查询

角度2：内查询是否被执行多次
	相关子查询  vs  不相关子查询
	
 比如：相关子查询的需求：查询工资大于本部门平均工资的员工信息。（各部门平均工资不同且依赖于部门员工）
       不相关子查询的需求：查询工资大于本公司平均工资的员工信息。（本公司平均工资是确定的一个数）
       结合概念理解
*/

2. 单行子查询

2.1 单行比较操作符

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2.2 代码示例

题目：查询工资大于149号员工工资的员工的信息

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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

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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

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SELECT last_name, job_id, salary
FROM employees
WHERE salary =
(SELECT MIN(salary)
FROM employees);
/*
+-----------+----------+---------+
| last_name | job_id   | salary  |
+-----------+----------+---------+
| Olson     | ST_CLERK | 2100.00 |
+-----------+----------+---------+
*/

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

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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

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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);
2.3 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

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SELECT	department_id, MIN(salary)
FROM	employees
GROUP BY department_id
HAVING	MIN(salary) >
(SELECT MIN(salary)
FROM	employees
WHERE	department_id = 50);
2.4 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’。

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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;
2.5 子查询中的空值问题
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SELECT last_name, job_id
FROM employees
WHERE job_id =
(SELECT job_id
FROM employees
WHERE last_name = 'Haas');
# Empty set (0.00 sec)
#子查询不返回任何行
2.5 非法使用子查询
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SELECT employee_id, last_name
FROM employees
WHERE salary =
(SELECT MIN(salary)
FROM employees
GROUP BY department_id);
#多行子查询使用单行比较符

多行子查询使用单行比较符

演示代码
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#子查询的编写技巧（或步骤）：① 从里往外写  ② 从外往里写

#4. 单行子查询
#4.1 单行操作符： =  !=  >   >=  <  <= 
#题目：查询工资大于149号员工工资的员工的信息
SELECT employee_id,last_name,salary
FROM employees
WHERE salary > (
		SELECT salary
		FROM employees
		WHERE employee_id = 149
		);
/*
+-------------+-----------+----------+
| employee_id | last_name | salary   |
+-------------+-----------+----------+
|         100 | King      | 24000.00 |
|         101 | Kochhar   | 17000.00 |
|         102 | De Haan   | 17000.00 |
|         108 | Greenberg | 12000.00 |
|         114 | Raphaely  | 11000.00 |
|         145 | Russell   | 14000.00 |
|         146 | Partners  | 13500.00 |
|         147 | Errazuriz | 12000.00 |
|         148 | Cambrault | 11000.00 |
|         168 | Ozer      | 11500.00 |
|         174 | Abel      | 11000.00 |
|         201 | Hartstein | 13000.00 |
|         205 | Higgins   | 12000.00 |
+-------------+-----------+----------+
*/

#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
		);
/*
+-------------+----------+---------+
| last_name   | job_id   | salary  |
+-------------+----------+---------+
| Nayer       | ST_CLERK | 3200.00 |
| Mikkilineni | ST_CLERK | 2700.00 |
| Bissot      | ST_CLERK | 3300.00 |
| Atkinson    | ST_CLERK | 2800.00 |
| Mallin      | ST_CLERK | 3300.00 |
| Rogers      | ST_CLERK | 2900.00 |
| Ladwig      | ST_CLERK | 3600.00 |
| Stiles      | ST_CLERK | 3200.00 |
| Seo         | ST_CLERK | 2700.00 |
| Rajs        | ST_CLERK | 3500.00 |
| Davies      | ST_CLERK | 3100.00 |
+-------------+----------+---------+
*/

#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
		);
/*
+-----------+----------+---------+
| last_name | job_id   | salary  |
+-----------+----------+---------+
| Olson     | ST_CLERK | 2100.00 |
+-----------+----------+---------+
*/

#题目：查询与141号员工的manager_id和department_id相同的其他员工
#的employee_id，manager_id，department_id。
#方式1：
SELECT employee_id,manager_id,department_id
FROM employees
WHERE manager_id = (
		    SELECT manager_id
		    FROM employees
		    WHERE employee_id = 141
		   )
AND department_id = (
		    SELECT department_id
		    FROM employees
		    WHERE employee_id = 141
		   )
AND employee_id <> 141;
/*
+-------------+------------+---------------+
| employee_id | manager_id | department_id |
+-------------+------------+---------------+
|         142 |        124 |            50 |
|         143 |        124 |            50 |
|         144 |        124 |            50 |
|         196 |        124 |            50 |
|         197 |        124 |            50 |
|         198 |        124 |            50 |
|         199 |        124 |            50 |
+-------------+------------+---------------+
*/

#方式2：了解
SELECT employee_id,manager_id,department_id
FROM employees
WHERE (manager_id,department_id) = (
				    SELECT manager_id,department_id
			            FROM employees
				    WHERE employee_id = 141
				   )
AND employee_id <> 141;



#题目：查询最低工资大于110号部门最低工资的部门id和其最低工资
SELECT department_id,MIN(salary)
FROM employees
WHERE department_id IS NOT NULL
GROUP BY department_id
HAVING MIN(salary) > (
			SELECT MIN(salary)
			FROM employees
			WHERE department_id = 110
		     );
/*
+---------------+-------------+
| department_id | MIN(salary) |
+---------------+-------------+
|            70 |    10000.00 |
|            90 |    17000.00 |
+---------------+-------------+
*/




#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;
/*部分输出 107行
+-------------+-------------+----------+
| employee_id | last_name   | location |
+-------------+-------------+----------+
|         100 | King        | USA      |
|         101 | Kochhar     | USA      |
|         102 | De Haan     | USA      |
|         103 | Hunold      | USA      |
|         104 | Ernst       | USA      |
|         105 | Austin      | USA      |
|         106 | Pataballa   | USA      |
*/


#4.2 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');#员工中没有Haas
                 
#4.3 非法使用子查询
#错误：Subquery returns more than 1 row
SELECT employee_id, last_name
FROM   employees
WHERE  salary =
                (SELECT   MIN(salary)
                 FROM     employees
                 GROUP BY department_id);#返回值不止一个，不能用等号连接

3. 多行子查询

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

请添加图片描述

3.2 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
请添加图片描述
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
请添加图片描述
题目：查询平均工资最低的部门id

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#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
)

3.3 空值问题
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SELECT last_name
FROM employees
WHERE employee_id NOT IN (
SELECT manager_id
FROM employees
);

演示代码
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#5.多行子查询
#5.1 多行子查询的操作符： IN  ANY ALL SOME(同ANY)
#IN 等于列表中的任意一个
#ANY 需要和单行比较操作符一起使用，和子查询返回的某一个值比较
#ALL 需要和单行比较操作符一起使用，和子查询返回的所有值比较
#SOME 实际上是ANY的别名，作用相同，一般常使用ANY
#5.2举例：
# IN:
SELECT employee_id, last_name
FROM   employees
WHERE  salary IN
                (SELECT   MIN(salary)
                 FROM     employees
                 GROUP BY department_id); 
/*26条记录
+-------------+------------+
| employee_id | last_name  |
+-------------+------------+
|         101 | Kochhar    |
|         102 | De Haan    |
|         104 | Ernst      |
|         107 | Lorentz    |
|         113 | Popp       |
|         119 | Colmenares |
*/                
                 
                 
                 
                 
# ANY / ALL:
#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 job_id <> 'IT_PROG'
AND salary < ANY (
		SELECT salary
		FROM employees
		WHERE job_id = 'IT_PROG'
		);
/*76条记录
+-------------+-------------+------------+---------+
| employee_id | last_name   | job_id     | salary  |
+-------------+-------------+------------+---------+
|         110 | Chen        | FI_ACCOUNT | 8200.00 |
|         111 | Sciarra     | FI_ACCOUNT | 7700.00 |
|         112 | Urman       | FI_ACCOUNT | 7800.00 |
|         113 | Popp        | FI_ACCOUNT | 6900.00 |
|         115 | Khoo        | PU_CLERK   | 3100.00 |
|         116 | Baida       | PU_CLERK   | 2900.00 |
*/		


#题目：返回其它job_id中比job_id为‘IT_PROG’部门所有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 job_id <> 'IT_PROG'
AND salary < ALL (
		SELECT salary
		FROM employees
		WHERE job_id = 'IT_PROG'
		);
/*4条记录
+-------------+-------------+----------+---------+
| employee_id | last_name   | job_id   | salary  |
+-------------+-------------+----------+---------+
|         115 | Khoo        | PU_CLERK | 3100.00 |
|         116 | Baida       | PU_CLERK | 2900.00 |
|         117 | Tobias      | PU_CLERK | 2800.00 |
|         118 | Himuro      | PU_CLERK | 2600.00 |
*/


		
#题目：查询平均工资最低的部门id
#注意：MySQL中聚合函数是不能嵌套使用的--如MIN(AVG(salary))是错误的
#注意：单行函数可以嵌套，Oracle中聚合函数可以嵌套
#方式1：
SELECT department_id 
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (
			SELECT MIN(avg_sal)
			FROM(
             #每一个子查询得到的表(AVG(salary))要有一个别名才可被外层查询使用			
				SELECT AVG(salary) avg_sal
				FROM employees
				GROUP BY department_id
				) t_dept_avg_sal
			);
/*
+---------------+
| department_id |
+---------------+
|            50 |
+---------------+
*/
#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
			); 
/*输出
+---------------+
| department_id |
+---------------+
|            50 |
+---------------+
*/		


		
#5.3 空值问题
SELECT last_name
FROM employees
WHERE employee_id NOT IN (
			SELECT manager_id
			FROM employees  
			);
#输出：Empty set (0.00 sec) 原因：内查询有一个空值NULL
#改正：
SELECT last_name
FROM employees
WHERE employee_id NOT IN (
			SELECT manager_id
			FROM employees WHERE manager_id IS NOT NULL
			);
/*
输出：
+-------------+
| last_name   |
+-------------+
| Ernst       |
| Austin      |
| Pataballa   |
*/

4. 相关子查询

4.1 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。
请添加图片描述
说明：子查询中使用主查询中的列

4.2 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
请添加图片描述
方式二：在 FROM 中使用子查询

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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

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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

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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);

4.3 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：

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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);

方式二：自连接

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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：

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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

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SELECT department_id, department_name
FROM departments d
WHERE NOT EXISTS (SELECT 'X'
FROM	employees
WHERE	department_id = d.department_id);
4.4 相关更新
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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

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# 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);
4.4 相关删除
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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

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DELETE FROM employees e
WHERE employee_id in
(SELECT employee_id
FROM	emp_history
WHERE	employee_id = e.employee_id);
演示代码
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#6. 相关子查询
#回顾：查询员工中工资大于公司平均工资的员工的last_name,salary和其department_id
#6.1 
SELECT last_name,salary,department_id
FROM employees
WHERE salary > (
		SELECT AVG(salary)
		FROM employees
		);#非相关子查询
		
#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1：使用相关子查询
SELECT last_name,salary,department_id
FROM employees e1
WHERE salary > (
		SELECT AVG(salary)
		FROM employees e2
		WHERE department_id = e1.`department_id`#保证本部门
		);#e1构成了相关性
/*输出
+-----------+----------+---------------+
| last_name | salary   | department_id |
+-----------+----------+---------------+
| King      | 24000.00 |            90 |
| Hunold    |  9000.00 |            60 |
| Ernst     |  6000.00 |            60 |
*/

#方式2（非相关子查询）：在FROM中声明子查询
SELECT e.last_name,e.salary,e.department_id
FROM employees e,(
		SELECT department_id,AVG(salary) avg_sal
		FROM employees
		GROUP BY department_id) t_dept_avg_sal
WHERE e.department_id = t_dept_avg_sal.department_id
AND e.salary > t_dept_avg_sal.avg_sal
/*
+-----------+----------+---------------+
| last_name | salary   | department_id |
+-----------+----------+---------------+
| Hartstein | 13000.00 |            20 |
| Raphaely  | 11000.00 |            30 |
| Weiss     |  8000.00 |            50 |
| Fripp     |  8200.00 |            50 |
| Kaufling  |  7900.00 |            50 |
*/





#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`
	) ASC;
/*
+-------------+----------+
| employee_id | salary   |
+-------------+----------+
|         178 |  7000.00 |
|         205 | 12000.00 |
|         206 |  8300.00 |
|         200 |  4400.00 |
*/

#结论：在SELECT中，除了GROUP BY 和 LIMIT之外，其他位置都可以声明子查询！
/*
SELECT ....,....,....(存在聚合函数)
FROM ... (LEFT / RIGHT)JOIN ....ON 多表的连接条件 
(LEFT / RIGHT)JOIN ... ON ....
WHERE 不包含聚合函数的过滤条件
GROUP BY ...,....
HAVING 包含聚合函数的过滤条件
ORDER BY ....,...(ASC / DESC )
LIMIT ...,....
*/






#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 *
FROM job_history;
/*
+-------------+------------+------------+------------+---------------+
| employee_id | start_date | end_date   | job_id     | department_id |
+-------------+------------+------------+------------+---------------+
|         101 | 1989-09-21 | 1993-10-27 | AC_ACCOUNT |           110 |
|         101 | 1993-10-28 | 1997-03-15 | AC_MGR     |           110 |
|         102 | 1993-01-13 | 1998-07-24 | IT_PROG    |            60 |
|         114 | 1998-03-24 | 1999-12-31 | ST_CLERK   |            50 |
|         122 | 1999-01-01 | 1999-12-31 | ST_CLERK   |            50 |
|         176 | 1998-03-24 | 1998-12-31 | SA_REP     |            80 |
|         176 | 1999-01-01 | 1999-12-31 | SA_MAN     |            80 |
|         200 | 1987-09-17 | 1993-06-17 | AD_ASST    |            90 |
|         200 | 1994-07-01 | 1998-12-31 | AC_ACCOUNT |            90 |
|         201 | 1996-02-17 | 1999-12-19 | MK_REP     |            20 |
+-------------+------------+------------+------------+---------------+
*/
SELECT employee_id,last_name,job_id
FROM employees e
WHERE 2 <= (
	    SELECT COUNT(*)
	    FROM job_history j
	    WHERE e.`employee_id` = j.`employee_id`
		)
/*输出：
+-------------+-----------+---------+
| employee_id | last_name | job_id  |
+-------------+-----------+---------+
|         101 | Kochhar   | AD_VP   |
|         176 | Taylor    | SA_REP  |
|         200 | Whalen    | AD_ASST |
+-------------+-----------+---------+
*/




#6.2 EXISTS 与 NOT EXISTS关键字
#题目：查询公司管理者的employee_id，last_name，job_id，department_id信息
#方式1：自连接
SELECT DISTINCT mgr.employee_id,mgr.last_name,mgr.job_id,mgr.department_id
FROM employees emp JOIN employees mgr
ON emp.manager_id = mgr.employee_id;
/*18行
+-------------+-----------+---------+---------------+
| employee_id | last_name | job_id  | department_id |
+-------------+-----------+---------+---------------+
|         100 | King      | AD_PRES |            90 |
|         101 | Kochhar   | AD_VP   |            90 |
|         102 | De Haan   | AD_VP   |            90 |
|         103 | Hunold    | IT_PROG |            60 |
*/

#方式2：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3：使用EXISTS
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`
	     );





#使用NOT EXISTS
#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1：
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;
/*
+---------------+----------------------+
| department_id | department_name      |
+---------------+----------------------+
|           120 | Treasury             |
|           130 | Corporate Tax        |
|           140 | Control And Credit   |
|           150 | Shareholder Services |
*/

#方式2：
SELECT department_id,department_name
FROM departments d
WHERE NOT EXISTS (
		SELECT *
		FROM employees e
		WHERE d.`department_id` = e.`department_id`
		);

SELECT COUNT(*)
FROM departments;

5. 抛一个思考题

问题：谁的工资比Abel的高？
解答：

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#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。

课后练习题

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# 第09章_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
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,manager_id
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1：
SELECT *
FROM departments
WHERE department_id = (
			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
							) t_dept_avg_sal

						)
			);
#方式2：ALL
SELECT *
FROM departments
WHERE department_id = (
			SELECT department_id
			FROM employees
			GROUP BY department_id
			HAVING AVG(salary ) <= ALL(
						SELECT AVG(salary)
						FROM employees
						GROUP BY department_id
						)
			);

#方式3： LIMIT

SELECT *
FROM departments
WHERE department_id = (
			SELECT department_id
			FROM employees
			GROUP BY department_id
			HAVING AVG(salary ) =(
						SELECT AVG(salary) avg_sal
						FROM employees
						GROUP BY department_id
						ORDER BY avg_sal ASC
						LIMIT 1		
						)
			);

#方式4：
SELECT d.*
FROM departments d,(
		SELECT department_id,AVG(salary) avg_sal
		FROM employees
		GROUP BY department_id
		ORDER BY avg_sal ASC
		LIMIT 0,1
		) 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1：
SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal
FROM departments d
WHERE department_id = (
			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
							) t_dept_avg_sal

						)
			);

#方式2：

SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal
FROM departments d
WHERE department_id = (
			SELECT department_id
			FROM employees
			GROUP BY department_id
			HAVING AVG(salary ) <= ALL(
						SELECT AVG(salary)
						FROM employees
						GROUP BY department_id
						)
			);

#方式3： LIMIT

SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal
FROM departments d
WHERE department_id = (
			SELECT department_id
			FROM employees
			GROUP BY department_id
			HAVING AVG(salary ) =(
						SELECT AVG(salary) avg_sal
						FROM employees
						GROUP BY department_id
						ORDER BY avg_sal ASC
						LIMIT 1		
						)
			);

#方式4：

SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal
FROM departments d,(
		SELECT department_id,AVG(salary) avg_sal
		FROM employees
		GROUP BY department_id
		ORDER BY avg_sal ASC
		LIMIT 0,1
		) 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1：
SELECT *
FROM jobs
WHERE job_id = (
		SELECT job_id
		FROM employees
		GROUP BY job_id
		HAVING AVG(salary) = (
					SELECT MAX(avg_sal)
					FROM (
						SELECT AVG(salary) avg_sal
						FROM employees
						GROUP BY job_id
						) t_job_avg_sal
					)
		);

#方式2：
SELECT *
FROM jobs
WHERE job_id = (
		SELECT job_id
		FROM employees
		GROUP BY job_id
		HAVING AVG(salary) >= ALL(
				     SELECT AVG(salary) 
				     FROM employees
				     GROUP BY job_id
				     )
		);

#方式3：
SELECT *
FROM jobs
WHERE job_id = (
		SELECT job_id
		FROM employees
		GROUP BY job_id
		HAVING AVG(salary) =(
				     SELECT AVG(salary) avg_sal
				     FROM employees
				     GROUP BY job_id
				     ORDER BY avg_sal DESC
				     LIMIT 0,1
				     )
		);

#方式4：
SELECT j.*
FROM jobs j,(
		SELECT job_id,AVG(salary) avg_sal
		FROM employees
		GROUP BY job_id
		ORDER BY avg_sal DESC
		LIMIT 0,1		
		) t_job_avg_sal
WHERE j.job_id = t_job_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1：自连接  xxx worked for yyy
SELECT DISTINCT mgr.employee_id,mgr.last_name,mgr.job_id,mgr.department_id
FROM employees emp JOIN employees mgr
ON emp.manager_id = mgr.employee_id;

#方式2：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3：使用EXISTS
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`
	     );

	
#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1：
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
						)
			);

SELECT *
FROM employees
WHERE department_id = 10;

#方式2：
SELECT MIN(salary)
FROM employees
WHERE department_id = (
			SELECT department_id
			FROM employees
			GROUP BY department_id
			HAVING MAX(salary) <= ALL (
						SELECT MAX(salary)
						FROM employees
						GROUP BY department_id
						)
			);

#方式3：
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 ASC
						LIMIT 0,1
						)
			);
			
#方式4：
SELECT MIN(salary)
FROM employees e,(
		SELECT department_id,MAX(salary) max_sal
		FROM employees
		GROUP BY department_id
		ORDER BY max_sal ASC
		LIMIT 0,1
		) t_dept_max_sal
WHERE e.department_id = t_dept_max_sal.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1：
SELECT last_name, department_id, email, salary
FROM employees
WHERE employee_id = ANY (
			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2：
SELECT last_name, department_id, email, salary
FROM employees
WHERE employee_id = ANY (
			SELECT DISTINCT manager_id
			FROM employees
			WHERE department_id = (
						SELECT department_id
						FROM employees
						GROUP BY department_id
						HAVING AVG(salary) >= ALL (
								SELECT AVG(salary) avg_sal
								FROM employees
								GROUP BY department_id
								)
						)
			);

#方式3：
SELECT last_name, department_id, email, salary
FROM employees
WHERE employee_id IN (
			SELECT DISTINCT manager_id
			FROM employees e,(
					SELECT department_id,AVG(salary) avg_sal
					FROM employees
					GROUP BY department_id
					ORDER BY avg_sal DESC
					LIMIT 0,1
					) t_dept_avg_sal
			WHERE e.`department_id` = t_dept_avg_sal.department_id
			);


#15. 查询部门的部门号，其中不包括job_id是"ST_CLERK"的部门号
#方式1：
SELECT department_id
FROM departments
WHERE department_id NOT IN (
			SELECT DISTINCT department_id
			FROM employees
			WHERE job_id = 'ST_CLERK'
			);

#方式2：
SELECT department_id
FROM departments d
WHERE NOT EXISTS (
		SELECT *
		FROM employees e
		WHERE d.`department_id` = e.`department_id`
		AND e.`job_id` = 'ST_CLERK'
		);



#16. 选择所有没有管理者的员工的last_name

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1：
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2：
SELECT employee_id,last_name,hire_date,salary
FROM employees e1
WHERE EXISTS (
		SELECT *
		FROM employees e2
		WHERE e1.`manager_id` = e2.`employee_id`
		AND e2.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（相关子查询）
#方式1：使用相关子查询
SELECT last_name,salary,department_id
FROM employees e1
WHERE salary > (
		SELECT AVG(salary)
		FROM employees e2
		WHERE department_id = e1.`department_id`
		);

#方式2：在FROM中声明子查询
SELECT e.last_name,e.salary,e.department_id
FROM employees e,(
		SELECT department_id,AVG(salary) avg_sal
		FROM employees
		GROUP BY department_id) t_dept_avg_sal
WHERE e.department_id = t_dept_avg_sal.department_id
AND e.salary > t_dept_avg_sal.avg_sal


#19.查询每个部门下的部门人数大于 5 的部门名称（相关子查询）
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（相关子查询）
SELECT * FROM locations;

SELECT country_id
FROM locations l
WHERE 2 < (
	   SELECT COUNT(*)
	   FROM departments d
	   WHERE l.`location_id` = d.`location_id`
	 );

/* 
子查询的编写技巧（或步骤）：① 从里往外写  ② 从外往里写

如何选择？
① 如果子查询相对较简单，建议从外往里写。一旦子查询结构较复杂，则建议从里往外写
② 如果是相关子查询的话，通常都是从外往里写。

*/