第09章_子查询

第09章_子查询

1.需求分析与解决问题

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。

1.1实际问题

需求:谁的工资比Abel的工资高?

现有解决方式:

#方式1:
SELECT last_name,salary
FROM employees
WHERE last_name = 'Abel';

SELECT last_name, salary
FROM employees
WHERE salary > 11000;

#方式2:自连接
SELECT e2.last_name, e2.salary
FROM employees e1, employees e2
WHERE e1.salary < e2.`salary`
AND e1.`last_name` LIKE 'Abel';



#方式3: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:

  • 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 。

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。

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 t

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

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);

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');

子查询不返回任何行

2.6 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);

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

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

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

题目:查询平均工资最低的部门id

#方式1SELECT 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2SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) <= ALL (
SELECT AVG(salary) avg_sal
FROM employees
GROUP BY department_id
)S

3.3空值问题

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

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。

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

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);

4.3 EXISTS 与NOTEXISTS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 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:

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_name4.4 相关更新

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

4.5 相关删除

5.抛出一个思考问题

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。

本章练习

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 = (
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 = ANY(
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
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(dept_avgsal)
FROM (
SELECT AVG(salary) dept_avgsal
FROM employees
GROUP BY department_id
) avg_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_sal
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_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal
LIMIT 0,1
)
)

#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
LIMIT 0,1) dept_avg_sal
WHERE d.department_id = dept_avg_sal.department_

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)
avg_sal
FROM departments d
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (
SELECT MIN(dept_avgsal)
FROM (
SELECT AVG(salary) dept_avgsal
FROM employees
GROUP BY department_id
) avg_sal
)
);

#方式二:
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) avg_sal
FROM employees
GROUP BY department_id
)
);
#方式三:
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
LIMIT 0,1
)
)

#方式四:
SELECT d.*,dept_avg_sal.avg_sal
FROM departments d,(
    SELECT department_id,AVG(salary) avg_sal
    FROM employees
    GROUP BY department_id
    ORDER BY avg_sal
    LIMIT 0,1) dept_avg_sal
    WHERE d.department_id = 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_sal)
        FROM(
        SELECT AVG(salary) avg_sal
        FROM employees
        GROUP BY job_id
        ) job_avgsal
        )
    );
    
    #方式二:
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
)
);

#方式三:
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
            )
	);


#方式四:
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 ) job_avg_sal
WHERE j.job_id = 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1SELECT employee_id,last_name,salary
FROM employees
WHERE employee_id IN (
SELECT DISTINCT manager_id
FROM employees
);

#方式2SELECT DISTINCT e1.employee_id, e1.last_name, e1.salary
FROM employees e1 JOIN employees e2
WHERE e1.employee_id = e2.manager_id;

#方式3SELECT employee_id, last_name, salary
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) 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) max_sal
FROM employees
GROUP BY department_id
)
);

#方式3SELECT 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 0,1
        )
	)
#方式4SELECT employee_id,MIN(salary)
FROM employees e,
(SELECT department_id,MAX(salary) max_sal
FROM employees
GROUP BY department_id
ORDER BY max_sal
LIMIT 0,1) dept_max_sal
WHERE e.department_id = 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

#方式一:
SELECT employee_id,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
) dept_sal
)
)
);

#方式二:
SELECT employee_id,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 e
GROUP BY department_id
HAVING AVG(salary)>=ALL(
SELECT AVG(salary)
FROM employees
GROUP BY department_id
)
)
);

#方式三:
SELECT *
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) dept_avg_sal
WHERE e.department_id = 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

#方法一:
SELECT department_id
FROM departments d
WHERE department_id NOT IN (
SELECT DISTINCT department_id
FROM employees
WHERE job_id = 'ST_CLERK'
);

#方法二:
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

SELECT last_name
FROM employees e1
WHERE NOT EXISTS (
SELECT *
FROM employees e2
WHERE e1.manager_id = e2.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1SELECT employee_id, last_name, hire_date, salary
FROM employees
WHERE manager_id = (
SELECT employee_id
FROM employees
WHERE last_name = 'De Haan'
)

#方式2SELECT employee_id, last_name, hire_date, salary
FROM employees e1
WHERE EXISTS (
SELECT *
FROM employees e2
WHERE e2.`employee_id` = e1.manager_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`
);
posted @   天覆者  阅读(45)  评论(0编辑  收藏  举报
相关博文:
阅读排行:
· PowerShell开发游戏 · 打蜜蜂
· 在鹅厂做java开发是什么体验
· 百万级群聊的设计实践
· WPF到Web的无缝过渡:英雄联盟客户端的OpenSilver迁移实战
· 永远不要相信用户的输入:从 SQL 注入攻防看输入验证的重要性
点击右上角即可分享
微信分享提示