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 salary
FROM employees
WHERE 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
| 操作符 | 含义 |
|---|---|
| = | equal to |
| > | greater than |
| >= | greater than or equal to |
| < | less than |
| <= | less than or equal to |
| <> | not equal to |
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_id FROM employees WHERE employee_id = 141) AND salary > (SELECT salary FROM employees WHERE 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_id FROM employees WHERE employee_id IN (174,141)) AND department_id IN (SELECT department_id FROM employees WHERE 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_id FROM employees WHERE 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 employees
WHERE 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_id
WHEN
(SELECT department_id FROM departments
WHERE 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_id
FROM employees
WHERE 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 employees
GROUP 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
| 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 |
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_sal
FROM employees
GROUP 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_sal
FROM employees
GROUP BY department_id
)
3、空值问题
SELECT last_name
FROM employees
WHERE employee_id NOT IN (
SELECT manager_id
FROM 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_name FROM departments d WHERE 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_idSELECT 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
- 条件返回
-
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 e2 WHERE 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_id FROM 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_name from employees e right join departments d on e.department_id = d.department_id where e.department_id is null; # 方式二： SELECT department_id, department_name FROM departments d WHERE NOT EXISTS (SELECT 'X' FROM employees WHERE department_id = d.department_id);
4、相关更新
UPDATE table1 alias1
SET column = (SELECT expression
FROM table2 alias2
WHERE 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 d
WHERE e.department_id = d.department_id);
5、相关删除
DELETE FROM table1 alias1
WHERE column operator (SELECT expression
FROM table2 alias2
WHERE 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_id
FROM 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 salary FROM employees WHERE 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_id
FROM employees
);
# 方式三：
SELECT employee_id, last_name, job_id, department_id
FROM employees e1
WHERE EXISTS ( SELECT *
FROM employees e2
WHERE 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 mgr
WHERE emp.manager_id = mgr.employee_id
AND 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 d
WHERE l.location_id = d.`location_id`
)
