一、子查询定义
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。
二、子查询的基本使用
1、子查询（内查询）在主查询之前一次执行完成。
2、子查询的结果被主查询（外查询）使用。
3、注意事项：
（1）子查询要包含在括号内
（2）将子查询放在比较条件的右侧
（3）单行操作符对应单行子查询，多行操作符对应多行子查询
三、子查询分类
1、分类方式一
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
。
2、分类方式二
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。
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。
四、单行子查询
1、单行比较操作符
举例：
# 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 salary
FROM employees
WHERE employee_id=149);
# 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);
# 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);
# 查询与141号员工的manager_id和department_id相同的其他员工
# 法一：(正常思路)
SELECT employee_id,manager_id,department_id
FROM employees
WHERE manager_id=(
SELECT manager_id
FROM employees
WHERE employee_id=141)
AND department_id=(
SELECT department_id
FROM employees
WHERE employee_id=141)
AND employee_id<>141;# 注意去除141号员工本身
# 法二：
SELECT employee_id,manager_id,department_id
FROM employees
WHERE (manager_id,department_id)=(
SELECT manager_id,department_id
FROM employees
WHERE employee_id=141)
AND employee_id<>141;# 注意去除141号员工本身
2、HAVING 中的子查询
#首先执行子查询，再向主查询中的HAVING 子句返回结果。
# 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 # 只能想到使用group by...having,因为where中不能使用聚合函数
HAVING MIN(salary)>(
SELECT MIN(salary)
FROM employees
WHERE department_id=50);
3、CASE中的子查询
# 在CASE表达式中使用单列子查询
# 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 department_id WHEN (SELECT department_id FROM departments WHERE location_id = 1800) THEN 'Canada'
ELSE 'USA' END "location" # CASE...WHEN...THEN...用在第一行
FROM employees;
4、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');# 公司没有Haas这个人，子查询返回空值，最终结果也是空值，但不会报错
5、非法使用子查询
# 报错： Subquery returns more than 1 row
# 原因：因为子查询的结果是多行数据，而父查询使用单行操作符=，不知道到底等于哪个数据
SELECT employee_id, last_name
FROM employees
WHERE salary =(
SELECT MIN(salary)
FROM employees
GROUP BY department_id);
五、多行子查询
1、多行比较操作符
举例：
# 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 salary< ANY(
SELECT salary
FROM employees
WHERE job_id='IT_PROG')
AND job_id<>'IT_PROG'; # 注意去除IT_PROG本身
# 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,salary
FROM employees
WHERE salary< ALL(
SELECT salary
FROM employees
WHERE job_id='IT_PROG')
AND job_id<>'IT_PROG'; # 注意去除IT_PROG本身
# 查询平均工资最低的部门id
# 法一
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (
SELECT MIN(avg_sal) # 将子查询出来的数据作为一个临时表，一定要为所查询的列取别名和临时表取表名
FROM(
SELECT AVG(salary) avg_sal
FROM employees
GROUP BY department_id
) t_dept_avg_sal
);
# 法二
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary)<=ALL( # 小于等于平均工资即相当于等于最小平均工资
SELECT AVG(salary)
FROM employees
GROUP BY department_id);
2、空值问题
SELECT last_name
FROM employees
WHERE employee_id NOT IN ( #返回结果为控制是因为子查询中查询出的manager_id有一个null值，导致not in判断有问题
SELECT manager_id
FROM employees
#WHERE manager_id IS NOT NULL
);
六、相关子查询
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。
举例：
# 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
# 法一：相关子查询
SELECT last_name,salary,department_id
FROM employees e1
WHERE salary>(
SELECT AVG(salary)
FROM employees e2
WHERE department_id=e1.department_id); # 注意“本部门”关键字眼
# 法二：在from中声明子查询
# 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。
SELECT e.last_name,e.salary,e.department_id #注意此处要标明department_id所属的表，不然会报错(Column 'department_id' in field list is ambiguous)
FROM employees e,(
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP 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;
# 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 # department_name在departmnets表中
FROM departments d
WHERE e.department_id=d.department_id);
# 重要结论：在SELECT中，除了GROUP BY 和 LIMIT之外，其他位置都可以声明子查询
# 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,e.last_name,e.job_id
FROM employees e
WHERE 2<=(SELECT COUNT(*)
FROM job_history
WHERE employee_id=e.employee_id);
2、EXISTS 与 NOT EXISTS关键字
（1）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。
（2）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。
（3）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。
（4）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。
举例：
# 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 e1.employee_id,e1.last_name,e1.job_id,e1.department_id # 因为管理者可能管理多个员工，所以需要去重
FROM employees e1 JOIN employees e2 # 相当于求交集
WHERE e1.employee_id=e2.manager_id;
# 方式二：子查询
SELECT employee_id,last_name,job_id,department_id
FROM employees
WHERE employee_id IN(
SELECT DISTINCT manager_id
FROM employees);
# 方式三：exists
SELECT e1.employee_id,e1.last_name,e1.job_id,e1.department_id
FROM employees e1
WHERE EXISTS(
SELECT *
FROM employees e2
WHERE e1.employee_id=e2.manager_id
);
# 查询departments表中，不存在于employees表中的部门的department_id和department_name
# 方式一：右连接
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; # employees表中没有的department_id
# 方式二：not exists(与上面一题方式三类似)
SELECT department_id,department_name
FROM departments d
WHERE NOT EXISTS (
SELECT *
FROM employees e
WHERE d.department_id= e.department_id
);
七、小练习
#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 (
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 IN (
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,manager_id
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1：
SELECT *
FROM departments
WHERE department_id = (
SELECT 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
) t_dept_avg_sal
)
);
#方式2：
SELECT *
FROM departments
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary ) <= ALL(
SELECT AVG(salary)
FROM employees
GROUP BY department_id
)
);
#方式3： LIMIT
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 ASC
LIMIT 1
)
);
#方式4：
SELECT d.*
FROM departments d,(
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal ASC
LIMIT 0,1
) 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1：
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(avg_sal)
FROM (
SELECT AVG(salary) avg_sal
FROM employees
GROUP BY department_id
) t_dept_avg_sal
)
);
#方式2：
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)
FROM employees
GROUP BY department_id
)
);
#方式3： LIMIT
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 ASC
LIMIT 1
)
);
#方式4：
SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal
FROM departments d,(
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal ASC
LIMIT 0,1
) t_dept_avg_sal
WHERE d.`department_id` = t_dept_avg_sal.department_id
#10.查询平均工资最高的 job 信息
#方式1：
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
) t_job_avg_sal
)
);
#方式2：
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3：
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4：
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
) t_job_avg_sal
WHERE j.job_id = t_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1：自连接 xxx worked for yyy
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_id
FROM employees
);
#方式3：使用EXISTS
SELECT employee_id,last_name,job_id,department_id
FROM employees e1
WHERE EXISTS (
SELECT *
FROM employees e2
WHERE e1.`employee_id` = e2.`manager_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
) t_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)
FROM employees
GROUP BY department_id
)
);
#方式3：
SELECT 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 ASC
LIMIT 0,1
)
);
#方式4：
SELECT MIN(salary)
FROM employees e,(
SELECT department_id,MAX(salary) max_sal
FROM employees
GROUP BY department_id
ORDER BY max_sal ASC
LIMIT 0,1
) t_dept_max_sal
WHERE e.department_id = t_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1：
SELECT last_name, department_id, email, salary
FROM employees
WHERE employee_id = ANY (
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
) t_dept_avg_sal
)
)
);
#方式2：
SELECT last_name, department_id, email, salary
FROM employees
WHERE employee_id = ANY (
SELECT DISTINCT manager_id
FROM employees
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3：
SELECT last_name, department_id, email, salary
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
) t_dept_avg_sal
WHERE e.`department_id` = t_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
#方式1：
SELECT department_id
FROM departments
WHERE department_id NOT IN (
SELECT DISTINCT department_id
FROM employees
WHERE job_id = 'ST_CLERK'
);
#方式2：
SELECT department_id
FROM departments d
WHERE NOT EXISTS (
SELECT *
FROM employees e
WHERE d.`department_id` = e.`department_id`
AND e.`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 emp
WHERE NOT EXISTS (
SELECT *
FROM employees mgr
WHERE 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1：
SELECT employee_id,last_name,hire_date,salary
FROM employees
WHERE manager_id IN (
SELECT employee_id
FROM employees
WHERE last_name = 'De Haan'
);
#方式2：
SELECT employee_id,last_name,hire_date,salary
FROM employees e1
WHERE EXISTS (
SELECT *
FROM employees e2
WHERE 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）
#方式1：使用相关子查询
SELECT last_name,salary,department_id
FROM employees e1
WHERE salary > (
SELECT AVG(salary)
FROM employees e2
WHERE department_id = e1.`department_id`
);
#方式2：在FROM中声明子查询
SELECT e.last_name,e.salary,e.department_id
FROM employees e,(
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP 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 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 * FROM locations;
SELECT country_id
FROM locations l
WHERE 2 < (
SELECT COUNT(*)
FROM departments d
WHERE l.`location_id` = d.`location_id`
);
/*
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。
*/
所用数据库请看文章末尾：
SQL语言和基本的select语句
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