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计算机等级培训机构,海淀区seo引擎优化,购物网站后台怎么做,政府门户网站有哪些功能文章目录 什么是子查询需求分析与问题解决子查询的基本语法结构子查询的分类 单行子查询单行比较操作符代码示例HAVING 中的子查询CASE中的子查询子查询中的空值问题非法使用子查询 多行子查询多行比较操作符代码示例空值问题 相关子查询代码示例在ORDER BY 中使用子查询EXISTS…

文章目录

  • 什么是子查询
    • 需求分析与问题解决
    • 子查询的基本语法结构
    • 子查询的分类
  • 单行子查询
    • 单行比较操作符
    • 代码示例
      • HAVING 中的子查询
      • CASE中的子查询
      • 子查询中的空值问题
      • 非法使用子查询
  • 多行子查询
    • 多行比较操作符
    • 代码示例
      • 空值问题
  • 相关子查询
    • 代码示例
      • 在ORDER BY 中使用子查询
      • EXISTS 与 NOT EXISTS关键字
    • 相关更新
    • 相关删除
  • 思考题
  • 练习

什么是子查询

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。

需求分析与问题解决

查询员工中谁的工资比Abel工资高

  • 这种问题，我们得先知道Abel的工资
#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 t1.salary
FROM employees t1 JOIN employees t2
WHERE t2.`last_name` = 'Abel' AND t1.salary > t2.`salary`;

子查询

select salary
from employees
where salary > (SELECT salaryFROM employeesWHERE last_name = 'Abel')

子查询的基本语法结构

image-20230924144922275
  • 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

  • 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 。

  • 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。

单行子查询

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 eq

单行子查询，也就是，内部查询的结果是唯一的

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 *
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 job_id,salary
FROM employees
WHERE salary > (SELECT salaryFROM employeesWHERE employee_id = 143)
AND job_id = (SELECT job_idFROM employeesWHERE employee_id = 141)

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

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#成对比较
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;

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 employeesWHERE department_id = 50)

CASE中的子查询

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_idWHEN (SELECT department_id FROM departments WHERE location_id = 1800) THEN 'Canada'ELSE 'USA'END)location
FROM employees

子查询中的空值问题

#空值问题
SELECT last_name, job_id
FROM employees
WHERE job_id =(	SELECT job_idFROM employeesWHERE last_name = 'Haas');

子查询不返回任何行

非法使用子查询

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

多行子查询使用单行比较符

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

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

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,salaryFROM employeesWHERE salary  < ANY(SELECT salaryFROM employeesWHERE job_id = 'IT_PROG')
AND 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

SELECT employee_id,last_name,job_id,salaryFROM employeesWHERE salary  < all(SELECT salaryFROM employeesWHERE job_id = 'IT_PROG')
AND job_id <> 'IT_PROG'

题目：查询平均工资最低的部门id

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

空值问题

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

相关子查询

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。

image-20230924221804718

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 t1.last_name,t1.salary,t1.department_id
FROM employees t1
WHERE salary > (SELECT mIN(salary)FROM employees t2WHERE t2.department_id = t1.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`;

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 t1
WHERE 2 <= (	SELECT COUNT(*)FROM job_history t2WHERE t1.employee_id = t2.employee_id)

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 t1
ORDER BY (	SELECT department_name FROM `departments` t2WHERE t2.`department_id` = t1.`department_id`) ASC

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

#使用非关联子查询
SELECT employee_id,last_name,job_id,department_id
FROM employees 
WHERE employee_id IN (SELECT DISTINCT manager_idFROM employees)
#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;
#使用EXISTS操作符
SELECT employee_id,last_name,job_id,department_id
FROM employees t1
WHERE EXISTS (SELECT 'A'FROM employees t2WHERE t1.`employee_id` = t2.`manager_id` )
  • 第一种写法in，执行过程中只有两次查询，先进行内部查询，然后根据内部查询的结果筛选外部条件，子查询会缓存到内存中，适合于子查询结果较小的情况，效率比较高，但是需要使用内存空间
  • 第三种exists，比较耗时，每次都是从外部查询中取出记录和内部查询匹配，内部的查询不会产生临时表，不耗费内存空间，适合于内部查询较大，且内存放不下的情况

题目：查询departments表中，不存在于employees表中的部门的department_id和department_name

SELECT department_id,department_name
FROM departments d1
WHERE NOT EXISTS (SELECT 'A'FROM employees t1WHERE d1.department_id = t1.`department_id`)

相关更新

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

ALTER TABLE employees
ADD(department_name VARCHAR(14));UPDATE employees e1
SET department_name = ( SELECT department_nameFROM departments d1WHERE e1.department_id = d1.department_id )

相关删除

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

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_historyWHERE employee_id = e.employee_id);

思考题

问题：谁的工资比Abel的高？

#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 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！
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 = (SELECT 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 last_name,salary,employee_id 
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 last_name,employee_id 
FROM `employees`
WHERE department_id IN  (SELECT department_idFROM employeesWHERE last_name LIKE '%u%')
SELECT last_name,employee_id 
FROM `employees`
WHERE department_id = any  (SELECT department_idFROM 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
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
SELECT last_name,salary
FROM employees
WHERE manager_id = ANY (SELECT employee_idFROM employeesWHERE 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
SELECT * 
FROM departments
WHERE department_id = (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) <= ALL (SELECT AVG(salary)FROM employeesGROUP BY department_id))
SELECT *
FROM departments
WHERE department_id = (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) = (SELECT MIN(dept_avgsal)FROM (SELECT AVG(salary) dept_avgsalFROM employeesGROUP BY department_id) avg_sal));
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_salLIMIT 0,1))	
SELECT d.*
FROM departments d,(SELECT department_id,AVG(salary) avg_salFROM employeesGROUP BY department_idORDER BY avg_salLIMIT 0,1) dept_avg_sal
WHERE d.department_id = 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）
SELECT d.*, (SELECT AVG(salary) FROM employees WHERE department_id = d.department_id ) minavg
FROM departments AS  d
HAVING d.department_id = (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) <= ALL (SELECT AVG(salary)FROM employeesGROUP BY department_id ))
SELECT d.*,dept_avg_sal.avg_sal
FROM departments d,(SELECT department_id,AVG(salary) avg_salFROM employeesGROUP BY department_idORDER BY avg_salLIMIT 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
SELECT *
FROM jobs
WHERE job_id = (SELECT job_idFROM employeesGROUP BY job_idHAVING AVG(salary) >= ALL(SELECT AVG(salary)FROM employeesGROUP BY job_id))
SELECT j.*
FROM jobs j,(SELECT job_id,AVG(salary) avg_salFROM employeesGROUP BY job_idORDER BY avg_sal DESCLIMIT 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 *
FROM departments
WHERE department_id IS NOT NULL
AND department_id IN (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) >(SELECT AVG(salary)FROM employees))
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_idFROM employeesWHERE manager_id IS NOT NULL)
#自连接
SELECT DISTINCT e1.employee_id, e1.last_name, e1.salary
FROM employees e1 JOIN employees e2
WHERE e1.employee_id = e2.manager_id;
#exist
SELECT * 
FROM employees e
WHERE EXISTS (SELECT 'A'FROM employeesWHERE manager_id = e.`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?
select min(salary)
from employees
where department_id = (select department_idfrom employeesgroup by department_idorder by Max(salary)limit 0,1)
SELECT MIN(salary)
FROM employees
WHERE department_id = (SELECT department_idFROM employeesGROUP BY department_idHAVING MAX(salary) = (SELECT MIN(max_sal)FROM (SELECT MAX(salary) max_salFROM employeesGROUP BY department_id) dept_max_sal));
SELECT employee_id,MIN(salary)
FROM employees e,(SELECT department_id,MAX(salary) max_salFROM employeesGROUP BY department_idORDER BY max_salLIMIT 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
SELECT *
FROM employees 
WHERE employee_id = (SELECT manager_idFROM departmentsWHERE department_id = (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) >= ALL (SELECT AVG(salary)FROM employeesGROUP BY 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
SELECT department_id
FROM departments d
WHERE department_id NOT IN (SELECT DISTINCT department_idFROM employeesWHERE job_id = 'ST_CLERK');
SELECT department_id
FROM departments d
WHERE  NOT EXISTS  (SELECT * FROM employees eWHERE 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 
where employee_id in (select employee_idfrom employeeswhere manager_id is null)
SELECT last_name
FROM employees e1
WHERE NOT EXISTS (SELECT *FROM employees e2WHERE 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'
SELECT employee_id, last_name, hire_date, salary
FROM employees
WHERE manager_id = (SELECT employee_idFROM employeesWHERE last_name = 'De Haan'
)
SELECT employee_id, last_name, hire_date, salary
FROM employees e1
WHERE EXISTS (SELECT * FROM employees e2WHERE 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）
SELECT e1.employee_id, e1.last_name, e1.salary
FROM employees e1
WHERE salary > (SELECT AVG(salary)FROM employees WHERE manager_id = e1.manager_id)
#方式二：
SELECT employee_id,last_name,salary
FROM employees e1,(	SELECT department_id,AVG(salary) avg_salFROM 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 eWHERE 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 dWHERE l.`location_id` = d.`location_id`
);
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