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

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'
		);

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1.2 子查询的基本使用

子查询的基本语法结构：

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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：

==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
- 多行子查询

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分类方式2：

==我们按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。

2. 单行子查询

2.1 单行比较操作符

操作符	含义
=	equal to
>	greater than
>=	greater than or equal to
<	less than
<=	less than or equal to
<>	not equal to

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

SELECT
	last_name,
	salary 
FROM
	employees 
WHERE
	salary > (
	SELECT
		salary 
	FROM
		employees 
	WHERE
	employee_id = 149 
	)

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 
	)

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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);

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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 ( 141, 174 ) ) 
	AND department_id IN (
	SELECT
		department_id 
	FROM
		employees 
	WHERE
	employee_id IN ( 141, 174 )) 
	AND department_id NOT IN (
	141,
	174)

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);

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

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’。

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

SELECT last_name, job_id
FROM   employees
WHERE  job_id =
                (SELECT job_id
                 FROM   employees
                 WHERE  last_name = 'Haas');

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子查询不返回任何行

2.5 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);

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子查询返回多条结果，而主查询使用=比较，就会报错。多行子查询使用单行比较符

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

操作符	含义
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

体会 ANY 和 ALL 的区别

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（小于最大值）

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")

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（小于最小值）

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")

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_salary ) FROM ( SELECT AVG( salary ) avg_salary FROM employees GROUP BY department_id ) t_avg_salary );

#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

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

                                                                                                                                                                     ![image-20211027195906773](https://imagebed-xuhuaiang.oss-cn-shanghai.aliyuncs.com/typora/image-20211027195906773.png)

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。

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

SELECT last_name, salary, department_id 
FROM employees e1 
WHERE salary > ( 
		SELECT AVG( salary ) 
		FROM employees e2 
		WHERE e1.department_id = e2.department_id );

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 ) DESC;

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 j
             WHERE  j.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： **

方式一：EXISTS

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

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：子查询

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：采用左外连接的方式

SELECT
	d.department_id,
	d.department_name 
FROM
	departments d
	LEFT JOIN employees e ON 
	d.department_id = e.department_id
WHERE e.department_id IS NULL;

方式二：采用NOT EXISTS

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

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4.4 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);

4.4 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);

5. 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。

6.章节练习

#1.查询和Zlotkey相同部门的员工姓名和工资
SELECT last_name,salary,department_id
FROM employees
WHERE department_id = (
	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
	job_id,
	last_name,
	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 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 e
	JOIN departments d ON e.department_id = d.department_id 
WHERE
	d.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 <= ALL (
	SELECT
		salary 
	FROM
		employees 
	);

#8.查询平均工资最低的部门信息

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 ) t_dept_avg_salary )
)



#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）
SELECT
	d.*,(SELECT AVG(salary) FROM employees e WHERE e.department_id = d.department_id) avg_salaary
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 ) t_dept_avg_salary )
);


#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( job_avg_salary ) FROM ( SELECT AVG( salary ) job_avg_salary FROM employees GROUP BY job_id ) job_avg ) 
	)
-- 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 ) job_avg_salary FROM employees GROUP BY job_id ) 
	)


#11.查询平均工资高于公司平均工资的部门有哪些?
SELECT
	* 
FROM
	departments 
WHERE
	department_id IN (
	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
SELECT
	* 
FROM
	employees 
WHERE
	employee_id IN ( SELECT DISTINCT manager_id FROM employees WHERE manager_id IS NOT NULL );

#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?
SELECT
	MIN( salary ) 
FROM
	employees 
WHERE
	department_id = (
	SELECT
		department_id 
	FROM
		employees 
	GROUP BY
		department_id 
	HAVING
		MIN( salary ) = ( SELECT MIN( max_salary ) FROM ( SELECT MAX( salary ) max_salary FROM employees GROUP BY department_id ) department_max_salary ) 
	);



#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
SELECT
	last_name,
	department_id,
	email,
	salary 
FROM
	employees 
WHERE
	employee_id = (
	SELECT
		manager_id 
	FROM
		departments 
	WHERE
		department_id = (
		SELECT
			department_id 
		FROM
			employees 
		GROUP BY
			department_id 
		HAVING
			AVG( salary ) = ( SELECT MAX( avg_salary ) FROM ( SELECT AVG( salary ) avg_salary FROM employees GROUP BY department_id ) department_avg_salary ) 
		) 
	);


#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:使用NOT EXISTS
SELECT
	d.department_id 
FROM
	departments d
WHERE NOT EXISTS (
	SELECT department_id
	FROM employees e
	WHERE e.department_id = d.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
WHERE manager_id IS NULL;

#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'
SELECT
	employee_id,
	last_name,
	hire_date,
	salary 
FROM
	employees 
WHERE
	manager_id IN (
	SELECT
		employee_id 
	FROM
		employees 
	WHERE
		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）

#方式一：相关子查询
SELECT
	employee_id,
	last_name,
	salary 
FROM
	employees e1 
WHERE
	salary > ( # 查询某员工所在部门的平均
	SELECT AVG( salary ) FROM employees e2 WHERE e2.department_id = e1.`department_id` );
#方式二：
SELECT employee_id,last_name,salary
FROM employees e1,
(SELECT department_id,AVG(salary) avg_sal
FROM employees e2 GROUP BY department_id
) dept_avg_sal
WHERE e1.`department_id` = dept_avg_sal.department_id
AND e1.`salary` > 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,
	department_id 
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
	country_id 
FROM
	locations l 
WHERE
	2 < ( SELECT COUNT(*) FROM departments d WHERE l.`location_id` = d.`location_id` );

MySQL

#MySQL #数据库

第09章_子查询

https://xhablog.online/2021/04/02/MySQL基础-第09章_子查询/

作者

Xu huaiang

发布于

2021年4月2日

许可协议

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