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第9章子查询

编程入门 行业动态 更新时间:2024-10-03 17:16:59

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第9章子查询

第9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。

问题引入

需求：谁的工资比Abel的高？

现有的解决方式：

#方式一：效率低，要进行两次数据交换，IO次数多
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 JOIN employees e2
ON e1.last_name = 'Abel' AND e2.salary > e1.salary;
#或
SELECT e2.last_name,e2.salary #注意这里都是从e2这张表中取
FROM employees e1,employees e2
WHERE e1.last_name = 'Abel' #多表的连接条件，非等值连接
AND e2.salary > e1.salary;#方式三：子查询
SELECT last_name,salary
FROM employees
WHERE salary > (SELECT salaryFROM employeesWHERE last_name = 'Abel');

子查询的使用

基本语法结构：

称谓的规范：外查询(或主查询)、内查询(或子查询)。

说明：

  • 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

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。

相关子查询与非相关子查询的区别：

​ 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。

举例：

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。
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。

单行子查询

单行比较操作符

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 salaryFROM employeesWHERE 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_idFROM employeesWHERE employee_id = 141)
AND salary > (SELECT salaryFROM employeesWHERE 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号员工的manager_id和department_id相同的其他员工的employee_id， manager_id，department_id

#方式一
SELECT employee_id,manager_id,department_id
FROM employees
WHERE manager_id = (SELECT manager_idFROM employeesWHERE employee_id = 141)
AND department_id = (SELECT department_idFROM employeesWHERE employee_id = 141)
AND employee_id <> 141;#方式二 (了解)
SELECT employee_id,manager_id,department_id
FROM employees
WHERE (manager_id,department_id) = (SELECT manager_id,department_idFROM employeesWHERE employee_id = 141)
AND employee_id <> 141;

方式二属于成对比较，这种情况实际中遇见的比较少，大多数情况用不到成对比较，仅作为了解。

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
WHERE department_id IS NOT NULL
GROUP BY department_id
HAVING MIN(salary) > (SELECT MIN(salary)FROM employeesWHERE department_id = 50);

CASE中的子查询

题目：显示员工的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 WHEN (SELECT department_idFROM departmentsWHERE location_id = 1800) THEN 'Canada'ELSE 'USA' END "location"
FROM employees;

子查询中的空值情况

子查询中的空值情况。原因是内查询的结果就是NULL，因此外查询返回的结果也是NULL。

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

子查询返回任何行。

非法使用子查询

SELECT employee_id,last_name
FROM employees
WHERE salary = (SELECT MIN(salary)FROM employeesGROUP BY department_id);

发生错误：Subquery returns more than 1 row。

= 是单行操作符，这个内查询返回的是多条记录，应该使用多行操作符。

多行子查询

1.也称为集合比较子查询

2.内查询返回多行
3.使用多行比较操作符

多行比较操作符

多行子查询的操作符：IN ANY ALL SOME(同ANY)。

单行子查询中 = 可以用 IN 替换，因为单行子查询中返回的结果只有一个。

多行子查询中IN可以替换成 = ANY，这里只谈可以实现，等下篇具体学优化问题。

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

注意体会 ANY 和 ALL 的区别

举例 IN

SELECT employee_id,last_name,salary
FROM employees
WHERE salary IN (SELECT MIN(salary)FROM employeesGROUP BY department_id);

举例 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 (		#注意题目中的任一SELECT salaryFROM employeesWHERE 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 (		#注意题目中的所有SELECT salaryFROM employeesWHERE job_id = 'IT_PROG');

题目：查询平均工资最低的部门id (有点难，两种解决方式需要好好理解)

#MySQL中聚合函数不支持嵌套
SELECT MIN(AVG(salary))	#错误的
FROM employees
GROUP BY department_id;#方式一
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (SELECT MIN(avg_sal)FROM (SELECT AVG(salary) avg_sal # 将结果作为新的表的一个字段，也得起别名FROM employeesGROUP BY department_id) t_dept_avg_sal	# 将查询的结果作为一张新的表，必须起别名);#方式二
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) <= ALL (SELECT AVG(salary) avg_salFROM employeesGROUP BY department_id);

空值问题

以下返回NULL

SELECT last_name
FROM employees
WHERE employee_id NOT IN (SELECT manager_idFROM employees#WHERE manager_id IS NOT NULL);

当内查询中含有NULL时，外查询的结果即为NULL。开发中需要注意。

相关子查询

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。

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：使用相关子查询
SELECT e1.last_name,e1.salary,e1.department_id
FROM employees e1
WHERE salary > (SELECT AVG(salary)FROM employees e2WHERE e2.department_id = e1.department_id);#方式二：在FROM中声明子查询
SELECT e1.last_name,e1.salary,e1.department_id
FROM employees e1,(SELECT department_id,AVG(salary) avg_sal 	#作为新表的字段，必须起别名FROM employees e2GROUP BY department_id) t_dept_avg_sal	 #作为一张新表，必须起别名
WHERE t_dept_avg_sal.department_id = e1.department_id
AND e1.salary > avg_sal;

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。必须起别名。

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_nameFROM departments dWHERE e.department_id = d.department_id) ASC;

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！

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 employee_id,last_name,job_id
FROM employees e
WHERE 2 <= (SELECT COUNT(*)FROM job_history jWHERE e.employee_id = j.employee_id);

EXISTS 与 NOT EXISTS关键字

注意：开发中一般可以用IN的地方都可以改写为EXISTS;可以用 NOT IN 的地方都可以改成NOT EXISTS。

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：自连接
SELECT DISTINCT mgr.employee_id,mgr.last_name,mgr.job_id,mgr.department_id
FROM employees emp,employees mgr
WHERE emp.manager_id = mgr.employee_id;#方式二：子查询
SELECT e1.employee_id,e1.last_name,e1.job_id,e1.department_id
FROM employees e1
WHERE employee_id IN (SELECT DISTINCT manager_idFROM employees e2);#方式三：使用EXISTS
SELECT employee_id,last_name,job_id,department_id
FROM employees e1
WHERE EXISTS (SELECT *FROM employees e2WHERE e1.employee_id = e2.manager_id);

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 *FROM employees eWHERE d.department_id = e.department_id);

相关更新

语法结构

UPDATE table1 alias1
SET column = (SELECT expressionFROM table2 alias2WHERE 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 dWHERE e.department_id = d.department_id);

相关删除

语法结构：

 DELETE FROM table1 alias1WHERE column operator (SELECT expressionFROM table2 alias2WHERE 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_idFROM emp_history WHERE employee_id = e.employee_id);

自连接 vs 子查询

问题：谁的工资比Abel的高？

方式一：自连接

SELECT e2.last_name,e2.salary 
FROM employees e1,employees e2 
WHERE e1.last_name = 'Abel' 
AND e1.salary < e2.salary;

方式二：子查询

SELECT last_name,salary 
FROM employees
WHERE salary > (SELECT salaryFROM employeesWHERE 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！

解释：

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。

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 (	#可能有多个名为Zlotkey的员工，所以用INSELECT department_idFROM employeesWHERE 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 salaryFROM employeesWHERE 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 #可以进行去重操作FROM employeesWHERE 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
WHERE department_id IN (SELECT department_idFROM departmentsWHERE location_id = 1700);

6.查询管理者是King的员工姓名和工资

#方式一
SELECT last_name,salary
FROM employees
WHERE manager_id IN (SELECT employee_idFROM employeesWHERE last_name = 'King');#方式二
SELECT e1.last_name,e1.salary
FROM employees e1
WHERE EXISTS (SELECT *FROM employees e2WHERE e1.manager_id = e2.employee_idAND e2.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 (从里往外一层一层分析)

#方式一
SELECT *
FROM departments
WHERE department_id = (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) = (SELECT MIN(avg_sal)FROM (SELECT AVG(salary) avg_salFROM employeesGROUP BY department_id) t_dept_avg_sal));#方式二
SELECT *
FROM departments
WHERE department_id = (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) <= ALL (SELECT AVG(salary)FROM employeesGROUP BY department_id));#方式三: LIMIT
SELECT *
FROM departments
WHERE department_id = (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) = (SELECT AVG(salary) avg_salFROM employeesGROUP BY department_idORDER BY avg_sal ASCLIMIT 1));#方式四
SELECT d.*
FROM departments d,(SELECT department_id,AVG(salary) avg_salFROM employeesGROUP BY department_idORDER BY avg_salLIMIT 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) avg_sal
FROM departments d
WHERE department_id = (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) = (SELECT MIN(avg_sal)FROM (SELECT AVG(salary) avg_salFROM employeesGROUP BY department_id) t_dept_avg_sal));#相应的方式二三四加上相同的字段即可
  1. 查询平均工资最高的 job 信息
#方式一
SELECT *
FROM jobs
WHERE job_id = (SELECT job_idFROM employeesGROUP BY job_idHAVING AVG(salary) = (SELECT MAX(avg_sal)FROM employees e,(SELECT AVG(salary) avg_salFROM employeesGROUP BY job_id) t_job_avg_sal));#方式二
SELECT *
FROM jobs
WHERE job_id = (SELECT job_idFROM employeesGROUP BY job_idHAVING AVG(salary) >= ALL (SELECT AVG(salary) avg_salFROM employeesGROUP BY job_id));#方式三
SELECT *
FROM jobs
WHERE job_id = (SELECT job_idFROM employeesGROUP BY job_idHAVING AVG(salary) =  (SELECT AVG(salary) avg_salFROM employeesGROUP BY job_idORDER BY avg_sal DESCLIMIT 1));#方式四
SELECT j.*
FROM jobs j,(SELECT job_id,AVG(salary) avg_salFROM employeesGROUP BY job_idORDER BY avg_sal DESCLIMIT 1) t_job_avg_sal
WHERE j.job_id = t_job_avg_sal.job_id;
  1. 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);
  1. 查询出公司中所有 manager 的详细信息.
#方式一：自连接
SELECT DISTINCT mgr.employee_id,mgr.last_name,mgr.job_id,mgr.department_id
FROM employees emp,employees mgr
WHERE emp.manager_id = mgr.employee_id;#方式二：子查询
SELECT e1.employee_id,e1.last_name,e1.job_id,e1.department_id
FROM employees e1
WHERE employee_id IN (SELECT DISTINCT manager_idFROM employees e2);#注意：开发中一般可以用IN的地方都可以改写为EXISTS#方式三：使用EXISTS
SELECT employee_id,last_name,job_id,department_id
FROM employees e1
WHERE EXISTS (SELECT *FROM employees e2WHERE e1.employee_id = e2.manager_id);
  1. 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 e.department_idFROM employees eGROUP BY department_idHAVING MAX(salary) = (SELECT MIN(max_sal)FROM (SELECT MAX(salary) max_salFROM employeesGROUP BY department_id) t_dept_max_sal));#方式二
SELECT MIN(salary)
FROM employees
WHERE department_id = (SELECT e.department_idFROM employees eGROUP BY department_idHAVING MAX(salary) <= ALL (SELECT MAX(salary) max_salFROM employeesGROUP BY department_id   ));#方式三
SELECT MIN(salary)
FROM employees
WHERE department_id = (SELECT e.department_idFROM employees eGROUP BY department_idHAVING MAX(salary) = (SELECT MAX(salary) max_salFROM employeesGROUP BY department_id ORDER BY max_sal ASCLIMIT 1));#方式四
SELECT MIN(salary)
FROM employees e,(SELECT department_id,MAX(salary) max_salFROM employeesGROUP BY department_id ORDER BY max_sal ASCLIMIT 1) t_dept_max_sal
WHERE e.department_id = t_dept_max_sal.department_id;
  1. 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_idFROM employeesWHERE department_id IN ( #IN可以替换成  = ANYSELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) = (SELECT MAX(avg_sal)FROM (SELECT AVG(salary) avg_salFROM employees																						  GROUP BY department_id																					) t_dept_avg_sal)));#方式二
SELECT last_name,department_id,email,salary
FROM employees
WHERE employee_id IN (SELECT DISTINCT manager_idFROM employeesWHERE department_id IN ( #IN可以替换成  = ANYSELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) >= ALL (SELECT AVG(salary) avg_sal																				FROM employeesGROUP BY department_id																					)));#方式三：使用LIMIT，同上#方式四：使用多表连接，同上
SELECT last_name,department_id,email,salary
FROM employees
WHERE employee_id IN (SELECT DISTINCT manager_idFROM employees e,(SELECT department_id,AVG(salary) avg_salFROM employeesGROUP BY department_idORDER BY avg_sal DESCLIMIT 1) t_dept_avg_salWHERE e.department_id = t_dept_avg_sal.department_id			);
  1. 查询部门的部门号，其中不包括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_idFROM employeesWHERE job_id = 'ST_CLERK');#出现 NOT IN 一般都可以替换成 NOT EXISTS
#方式二
SELECT department_id
FROM departments d
WHERE NOT EXISTS (SELECT *FROM employees eWHERE d.department_id = e.department_idAND job_id = 'ST_CLERK');
  1. 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 mgrWHERE 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_idFROM employeesWHERE last_name = 'De Haan');#方式二
SELECT employee_id,last_name,hire_date,salary
FROM employees emp
WHERE EXISTS (SELECT *FROM employees mgrWHERE emp.manager_id = mgr.employee_idAND 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

#方式一：相关子查询
SELECT e1.employee_id,e1.last_name,e1.salary
FROM employees e1
WHERE salary > (SELECT AVG(salary)FROM employees e2WHERE e1.department_id = e2.department_id)#方式二：FROM中声明子查询
SELECT e.employee_id,e.last_name,e.salary
FROM employees e,(SELECT department_id,AVG(salary) avg_salFROM employeesGROUP 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 eWHERE d.department_id = e.department_id);

20.查询每个地区下的部门个数大于 2 的国家编号(相关子查询)

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

子查询的编写技巧

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。

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