MySQL数据库入门到大蛇尚硅谷宋红康老师笔记基础篇 part 9

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。

现在有一个需求：谁的工资比Abel的高？
方式1：

SELECT 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 el,employees e2
WHERE e2.`salary` > el.`salary` #多表的连接条件
AND e1.last_name 'Abel';

#方式3：子查询

SELECT last _name,salary
FROM employees
WHERE salary>(SELECT salaryFROM employeesWHERE last_name= 'Abel');

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

3 子查询的分类

分类方式1：

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。

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

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

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
);

解决问题的方法是 , 先模仿那个分两步完成的 , 然后 ,套入格式即可

#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。

SELECT employee_id,manager_id,department_id
FROM employees
WHERE manager_id =(SELECT manager_idFROM employeesWHERE employee_id = (141,174) #in是多行的时候用的，这里被改成了=  }
AND department_id=(SELECT department_idFROM employeesWHERE employee_id =141)
AND employee_id <> 141;

#方式2：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;

HAVING 中的子查询

#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;#子查询，只能返回一行数据，返回多条查询语句会报错的
#格式化后如下
SELECT employee_id, last_name,(CASE department_idWHEN(SELECT department_id FROM departmentsWHERE location_id = 1800)           THEN 'Canada' ELSE 'USA' END) location
FROM   employees;

子查询中的空值问题

SELECT last_name, job_idFROM   employeesWHERE  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_nameFROM   employeesWHERE  salary =            #单行操作符不能操作多行结果集 用IN才对(SELECT   MIN(salary)FROM     employeesGROUP BY department_id);

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

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

in和any : in 代表等于某一个而any可以用大于/小于/等于某一个

当我们这题老师答案匹配的是工资等于各部门最低工资之一的员工 ,要想匹配各部门的最低工资员工可以这样做：将IN 和=替换 , 因为此时是一样的.

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 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,salaryFROM employeesWHERE job_id <> 'IT_PROG'AND salary < ALL (SELECT salaryFROM employeesWHERE job_id = 'IT_PROG');

#题目：查询平均工资最低的部门id

#MySQL中聚合函数是不能嵌套使用的。如图oracle就可以直接嵌套的

#方式1：
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) t_dept_avg_sal);#方式2：
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) <= ALL(	SELECT AVG(salary) avg_salFROM employeesGROUP BY department_id)

拆解题目就完事了，把能做的点做出来，然后换元法。

3.3 空值问题

在内查询中有一个null，那么在null的条件下，接着where等等判断就是一个空的情况了（与null运算都为null）

IN是与任一的值比较的，加了NOT就是与全部比了

4. 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。

说明：子查询中使用主查询中的列

可以理解为子查询是一个常数,而相关子查询的结果则为一个变量x

首先取外表的记录，然后把记录给他送进去，然后送进去的数据，用的（）里面的记录的哪一个字段，即内部查询开始计算了，所以叫EXEUCUTE，子查询的使用主查询的这个数据，运算完后返回一个select的这样的一个结果。第三步是第一步送的记录和第二步返回的这条记录呢去做一个比较，看看是否满足比较的条件。

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
其实这个子查询，就看成两层for循环，第一层每一个变量执行内层for的循环就行了

SELECT last_name, salary, department_id
FROM employees e1
WHERE salary > (SELECT AVG(salary)FROM employees e2WHERE department_id =e1 .`department_id`
);

方式二：在 FROM 中使用子查询方式二是sql92的内连接

SELECT last_name,salary,e1.department_idFROM employees e1,(SELECT department_id,AVG(salary) dept_avg_sal FROM employees GROUP 
BY department_id) e2WHERE e1.`department_id` = e2.department_idAND e2.dept_avg_sal < e1.`salary`;

题目：查询员工的id,salary,按照department_name 排序

 SELECT employee_id,salaryFROM employees eORDER BY (SELECT department_nameFROM departments dWHERE 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_historyWHERE employee_id = e.employee_id);SELECT *
FROM job_history;SELECT employee_id, last_name, job_id
FROM employees e
WHERE 2 <= (SELECT COUNT(*)FROM job_history jWHERE e.employee_id = j.employee_id
);

那么在哪些位置可以写子查询呢？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！

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;

#方式2：子查询
SELECT employee_id, last_name, job_id, department_id
FROM employees
WHERE employee_id IN (SELECT DISTINCT manager_idFROM employees
);

#方式3：使用EXISTS
SELECT employee_id, last_name, job_id, department_id
FROM employees e1
WHERE EXISTS (SELECT *FROM employees e2WHERE e1.manager_id = e2.employee_id
);

WHERE EXISTS子句的目的是检查是否至少存在一行满足条件，而不是返回所有匹配到的行。

2. 因此当符合连接条件的出现一对多时就会产生多条结果
3.在此例中就包含了 emp 和 mgr 表中都符合 emp.employee_id=mgr.magager_id 的结果集

题目：查询departments表中，不存在于employees表中的部门的department_id和department_name 46-相关子查询案例分析_哔哩哔哩_bilibili

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

4.4 相关更新

4.4 相关删除

5. 抛一个思考题问题：谁的工资比Abel的高？

课后练习

47-第9章子查询课后练习1_哔哩哔哩_bilibili
48-第9章子查询课后练习2_哔哩哔哩_bilibili

#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 e1.employee_id, e1.last_name
FROM employees e1
JOIN employees e2 ON e1.department_id = e2.department_id
WHERE e2.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 = (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 = (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
ORDER BY salary ASC
LIMIT 1;#8. 查询平均工资最低的部门信息
SELECT department_id, department_name
FROM departments
WHERE department_id = (SELECT department_id FROM (SELECT department_id, AVG(salary) AS avg_salaryFROM employeesGROUP BY department_id) AS subqueryORDER BY avg_salary ASCLIMIT 1
);#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.department_id, d.department_name, AVG(e.salary) AS avg_salary
FROM departments d
JOIN employees e ON d.department_id = e.department_id
GROUP BY d.department_id
ORDER BY avg_salary ASC
LIMIT 1;#10. 查询平均工资最高的job信息
SELECT job_id, job_title
FROM jobs
WHERE job_id = (SELECT job_id FROM (SELECT job_id, AVG(salary) AS avg_salaryFROM employeesGROUP BY job_id) AS subqueryORDER BY avg_salary DESCLIMIT 1
);#11. 查询平均工资高于公司平均工资的部门有哪些？
SELECT department_id, department_name
FROM departments
WHERE department_id IN (SELECT department_id FROM (SELECT department_id, AVG(salary) AS avg_salaryFROM employeesGROUP BY department_id) AS subqueryWHERE 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 job_id = 'MGR';#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(max_salary) AS min_max_salary
FROM (SELECT MAX(salary) AS max_salaryFROM employeesGROUP BY department_id
) AS subquery;#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 e.last_name, e.department_id, e.email, e.salary
FROM employees e
JOIN (SELECT department_id FROM (SELECT department_id, AVG(salary) AS avg_salaryFROM employeesGROUP BY department_id) AS subqueryORDER BY avg_salary DESCLIMIT 1
) AS d ON e.department_id = d.department_id AND e.job_id = 'MGR';#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
WHERE department_id NOT IN (SELECT department_id FROM employees WHERE 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 = (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）
SELECT e1.employee_id, e1.last_name, e1.salary
FROM employees e1
WHERE e1.salary > (SELECT AVG(e2.salary) FROM employees e2 WHERE e2.department_id = e1.department_id
);#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
WHERE department_id IN (SELECT department_id FROM (SELECT department_id, COUNT(*) AS num_employeesFROM employeesGROUP BY department_id) AS subqueryWHERE num_employees > 5
);#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 countries
WHERE country_id IN (SELECT country_id FROM (SELECT country_id, COUNT(*) AS num_departmentsFROM departmentsGROUP BY country_id) AS subqueryWHERE num_departments > 2
);