目录

• 子查询
• • 一、需求分析与问题解决
  • • • 1、实际问题
      • 2、子查询的基本使用
      • 3、子查询的分类
  • 二、单行子查询
  • • • 1、单行比较操作符
      • 2、 代码示例
      • 3、HAVING 中的子查询
      • 4、CASE中的子查询
      • 5、 子查询中的空值问题
      • 6、 非法使用子查询
  • 三、多行子查询
  • • • 1、多行比较操作符
      • 2、代码示例
      • 3、空值问题
  • 四、 相关子查询
  • • • 1、相关子查询执行流程
      • 2、代码示例
      • 3、EXISTS 与 NOT EXISTS关键字
      • 4、相关更新
      • 5、相关删除
  • 五、抛一个思考题
  • 六、子查询的课后练习

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。

一、需求分析与问题解决

1、实际问题

现有解决方式：

#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 e2.last_name,e2.salary
FROM employees e1,employees e2
WHERE e1.last_name = 'Abel'
AND e1.`salary` < e2.`salary`

#方式三：子查询
SELECT last_name,salary
FROM employees
WHERE salary > (SELECT salaryFROM employeesWHERE last_name = 'Abel');

2、子查询的基本使用

• 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

3、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。

  • 单行子查询 也可以包含一行多个列
    

  • 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。

  • 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。

二、单行子查询

1、单行比较操作符

2、 代码示例

• 题目：查询工资大于149号员工工资的员工的信息

• 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

  • 实现方式1：不成对比较

    SELECT  employee_id, manager_id, department_id
    FROM    employees
    WHERE   manager_id IN(SELECT  manager_idFROM    employeesWHERE   employee_id IN (174,141))
    AND     department_id IN (SELECT  department_idFROM    employeesWHERE   employee_id IN (174,141))
    AND	employee_id NOT IN(174,141);

  • 实现方式2：成对比较

    SELECT	employee_id, manager_id, department_id
    FROM	employees
    WHERE  (manager_id, department_id) IN(SELECT manager_id, department_idFROM   employeesWHERE  employee_id IN (141,174))
    AND	employee_id NOT IN (141,174);

3、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);

4、CASE中的子查询

在CASE表达式中使用单列子查询：

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 departmentsWHERE location_id = 1800)           THEN 'Canada' ELSE 'USA' END) location
FROM   employees;

5、 子查询中的空值问题

SELECT last_name, job_id
FROM   employees
WHERE  job_id =(SELECT job_idFROM   employeesWHERE  last_name = 'Haas');
# 语句解释：
#	因为数据中没有 last_name = 'Haas' 的数据，所以 job_id 为 null, 外层用 job_id = null去查询所以显示为null

子查询不返回任何行

6、 非法使用子查询

SELECT employee_id, last_name
FROM   employees
WHERE  salary =(SELECT   MIN(salary)FROM     employeesGROUP BY department_id);
# 语句解释：
#	因为子查询语句中 select min(salary) from employees group by department_id 查询出了多条数据
#	那么 salary 不知道使用哪个去等
# 怎么改？用多行子查询关键字 in 

多行子查询使用单行比较符

三、多行子查询

• 也称为集合比较子查询
• 内查询返回多行数据
• 使用多行比较操作符

1、多行比较操作符

体会 ANY 和 ALL 的区别 @TODO

2、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, salary 
FROM employees
WHERE salary < ANY (SELECT salary FROM employees WHERE 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

题目：查询平均工资最低的部门id

#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) 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
)

3、空值问题

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

四、 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。

说明：子查询中使用主查询中的列

2、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
  

• 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`;

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。

• 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`);

   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_history WHERE  employee_id = e.employee_id);

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
  

• 方式一：

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

• 方式二：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;

• 方式三：

  SELECT employee_id,last_name,job_id,department_id
  FROM employees
  WHERE employee_id IN (SELECT DISTINCT manager_idFROM employees);

   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_namefrom employees e right join departments don e.department_id = d.department_idwhere e.department_id is null;# 方式二：SELECT department_id, department_nameFROM departments dWHERE NOT EXISTS (SELECT 'X'FROM   employeesWHERE  department_id = d.department_id);

4、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

# 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);

5、相关删除

 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);

五、抛一个思考题

• 问题：谁的工资比Abel的高？

• 解答：

  #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。

六、子查询的课后练习

# 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
# 方式一：
SELECT employee_id
FROM employees e JOIN departments d ON e.department_id = d.department_id
WHERE d.location_id = '1700'
# 方式二：
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
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
# 方式一：
SELECT * FROM departments WHERE department_id = (SELECT department_id
FROM employees
GROUP BY department_id 
HAVING AVG(salary) = (
SELECT MIN(avg_salary)  
FROM 
(SELECT AVG(salary) avg_salary
FROM employees
GROUP BY department_id) vi_dept_sal))# 方式二
SELECT * FROM departments WHERE department_id = (SELECT department_id
FROM employees
GROUP BY department_id 
HAVING AVG(salary) <= ALL (
SELECT AVG(salary) avg_salary
FROM employees
GROUP BY department_id) )# 方式三：
SELECT * FROM departments WHERE department_id = (SELECT department_id
FROM employees
GROUP BY department_id 
HAVING AVG(salary) = (SELECT AVG(salary) avg_salary
FROM employees
GROUP BY department_id
ORDER BY avg_salary ASC
LIMIT 1))# 方式四：
SELECT d.* FROM departments d,(
SELECT department_id, AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal ASC
LIMIT 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) avf_sal
FROM departments d WHERE department_id = (SELECT department_id
FROM employees
GROUP BY department_id 
HAVING AVG(salary) = (
SELECT MIN(avg_salary)  
FROM 
(SELECT AVG(salary) avg_salary
FROM employees
GROUP BY department_id) vi_dept_sal))# 方式二
SELECT * ,(SELECT AVG(salary) FROM employees WHERE department_id = d.department_id) avf_sal
FROM departments d WHERE department_id = (SELECT department_id
FROM employees
GROUP BY department_id 
HAVING AVG(salary) <= ALL (
SELECT AVG(salary) avg_salary
FROM employees
GROUP BY department_id) )# 方式三：
SELECT * ,(SELECT AVG(salary) FROM employees WHERE department_id = d.department_id) avf_sal
FROM departments d WHERE department_id = (SELECT department_id
FROM employees
GROUP BY department_id 
HAVING AVG(salary) = (SELECT AVG(salary) avg_salary
FROM employees
GROUP BY department_id
ORDER BY avg_salary ASC
LIMIT 1))# 方式四：
SELECT d.* ,(SELECT AVG(salary) FROM employees WHERE department_id = d.department_id) avf_sal
FROM departments d,(
SELECT department_id, AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal ASC
LIMIT 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
# 方式一：
SELECT * FROM jobs WHERE job_id = (SELECT job_id
FROM employees
GROUP BY job_id 
HAVING AVG(salary) = (
SELECT MAX(avg_salary)  
FROM 
(SELECT AVG(salary) avg_salary
FROM employees
GROUP BY job_id) vi_dept_sal))# 方式二
SELECT * FROM jobs WHERE job_id = (SELECT job_id
FROM employees
GROUP BY job_id 
HAVING MAX(salary) >= ALL (
SELECT AVG(salary) avg_salary
FROM employees
GROUP BY job_id) )# 方式三：
SELECT * FROM jobs WHERE job_id = (SELECT job_id
FROM employees
GROUP BY job_id 
HAVING AVG(salary) = (SELECT AVG(salary) avg_salary
FROM employees
GROUP BY job_id
ORDER BY avg_salary DESC
LIMIT 1))# 方式四：
SELECT d.* FROM jobs d,(
SELECT job_id, AVG(salary) avg_sal
FROM employees
GROUP BY job_id
ORDER BY avg_sal DESC
LIMIT 1) t_dept_avg_sal
WHERE d.job_id = t_dept_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
# 方式一：自查询
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;# 方式二：子查询
SELECT employee_id,last_name,job_id,department_id
FROM employees
WHERE employee_id IN (SELECT DISTINCT manager_idFROM employees);
# 方式三：
SELECT employee_id, last_name, job_id, department_id
FROM   employees e1
WHERE  EXISTS ( SELECT *FROM   employees e2WHERE  e2.manager_id = e1.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？
# 方式一：
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 ))# 方式二：
SELECT MIN(salary) 
FROM employees 
WHERE department_id = (
SELECT department_id 
FROM employees
GROUP BY department_id
HAVING MAX(salary) <= ALL (
SELECT MAX(salary) max_sal
FROM employees
GROUP BY department_id ))# 方式三：
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
LIMIT 1))# 方式四：
SELECT MIN(salary)
FROM employees e, (SELECT department_id, MAX(salary) max_sal
FROM employees
GROUP BY department_id
ORDER BY max_sal
LIMIT 1) t_dept_max_sal
WHERE t_dept_max_sal.department_id = e.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
# 方式一：
SELECT last_name,department_id,email,salary
FROM employees 
WHERE employee_id IN (
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)))
# 方式二：（与上面同思路）
# 方式三：（与上面同思路）
# 方式四：（与上面同思路）# 15、查询部门的部门号，其中不包含 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_id FROM employees WHERE job_id = 'ST_CLERK')# 方式二：
SELECT department_id 
FROM departments d
WHERE NOT EXISTS (SELECT * FROM employees e WHERE 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
# 方式一：
SELECT last_name 
FROM employees
WHERE manager_id IS NULL# 方式二：
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'
# 方式一：
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（相关查询）
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`;# 19、查询每个部门下的部门人数大于 5 的部门名称（相关查询）
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（相关查询）SELECT country_id 
FROM locations l
WHERE 2 < (SELECT COUNT(*)FROM departments dWHERE l.location_id = d.`location_id`
)