初级篇—第五章子查询

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

• 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。

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`
);