09 【子查询】
09 【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。
先写结论:在SELECT中,除了GROUP BY 和 LIMIT之外,其他位置都可以声明子查询!
SELECT ....,....,....(存在聚合函数)
FROM ... (LEFT / RIGHT)JOIN ....ON 多表的连接条件
(LEFT / RIGHT)JOIN ... ON ....
WHERE 不包含聚合函数的过滤条件
GROUP BY ...,....
HAVING 包含聚合函数的过滤条件
ORDER BY ....,...(ASC / DESC )
LIMIT ...,....
1. 需求分析与问题解决
1.1 实际问题
![image-20221020194326690](https://i0.hdslb.com/bfs/album/f46772a19fabdf371cb29d41f2929732fdc225df.png)
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'
);
![image-20221020194344754](https://i0.hdslb.com/bfs/album/7893918b47af93594158c63026b4bf0bf344c3f6.png)
1.2 子查询的基本使用
- 子查询的基本语法结构:
![image-20221020194656348](https://i0.hdslb.com/bfs/album/10b70d32b3c261f0218f9df3c200334bcf28cc5e.png)
- 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
1.3 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
![image-20221020195259239](https://i0.hdslb.com/bfs/album/e473557403677fef0a1cd602224782e148f81c6e.png)
- 多行子查询
![image-20221020195327859](https://i0.hdslb.com/bfs/album/868174eb87bed7354694c994b16733727439204b.png)
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。
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 |
2.2 代码示例
题目:查询工资大于149号员工工资的员工的信息
![image-20221020200905859](https://i0.hdslb.com/bfs/album/5964c2447f81af4238e9738ec43d883682f7cad7.png)
![image-20221020200912862](https://i0.hdslb.com/bfs/album/5f870b496d335e5c99000b95bc1d09f9f9299efb.png)
题目:返回job_id与员工号为141的员工相同,salary比员工号为143的员工多的员工姓名,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);
![image-20221020202106504](https://i0.hdslb.com/bfs/album/020635abda8413c43bdbc04783c31850e84147d9.png)
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);
![image-20221020202216852](https://i0.hdslb.com/bfs/album/2edd76f8aaa3c3097e9a649a55510d1f2819ac10.png)
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);
2.3 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);
2.4 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;
2.5 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');
![image-20221020202956416](https://i0.hdslb.com/bfs/album/b5f9dcfe0ab5bbc0eeafe6bded2a0c728e5c1856.png)
子查询不返回任何行
2.6 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);
![image-20221020203056319](https://i0.hdslb.com/bfs/album/17b575ea99fa3608909ee5a608a31dad925287de.png)
多行子查询使用单行比较符
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
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
3.2 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
![image-20221020203140032](https://i0.hdslb.com/bfs/album/e679bd0fad5ac9936d05dc2952cf3fcabfc8e066.png)
![image-20221020203149809](https://i0.hdslb.com/bfs/album/023f4e25fcea5f21cfb64a2adba18f754e190299.png)
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
![image-20221020203205815](https://i0.hdslb.com/bfs/album/8e6b9eae3b72c421a4cbd67da138d81d8f5aa30f.png)
![image-20221020203916085](https://i0.hdslb.com/bfs/album/90426d7a663eebb0262535efa1a963ab8d9b3b41.png)
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
)
-- 方式3:
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id
ORDER BY AVG(salary) ASC
LIMIT 1
3.3 空值问题
SELECT last_name
FROM employees
WHERE employee_id NOT IN (
SELECT manager_id -- manager_id可能为空,not in(null,....)---->结果为空
FROM employees
);
![image-20221020204008643](https://i0.hdslb.com/bfs/album/2d4d95a1ff6ff1b95b83d39f51757dc18e654d8f.png)
4. 相关子查询
4.1 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。
![image-20221020204023748](https://i0.hdslb.com/bfs/album/f2bb2ab1986fa554c016bc3393c0cd2bafb2e477.png)
![image-20221020204029553](https://i0.hdslb.com/bfs/album/e17fa39989a1e1dc2c489a09b96f2b85ff9606ef.png)
说明:子查询中使用主查询中的列
4.2 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
![image-20221020204101384](https://i0.hdslb.com/bfs/album/cfa0f3ef9f79502749fd26c6669f8bf5726ef9c6.png)
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
-- 方式1
SELECT employee_id,salary
FROM employees e
ORDER BY (
SELECT department_name
FROM departments d
WHERE e.`department_id` = d.`department_id`
);
-- 方式2:
SELECT employee_id,salary,department_name
FROM employees
JOIN departments
ON employees.department_id = departments.department_id
ORDER BY department_name;
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);
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:
- 条件返回 TRUE
- 不在子查询中继续查找
- 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 manager
WHERE EXISTS ( SELECT *
FROM employees worker
WHERE worker.manager_id =
manager.employee_id);
方式二:自连接
SELECT DISTINCT manager.employee_id,manager.last_name,manager.job_id,manager.department_id
FROM employees as work
JOIN employees manager
ON work.manager_id = manager.employee_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
SELECT department_id, department_name
FROM departments d
WHERE NOT EXISTS (SELECT 'X'
FROM employees
WHERE department_id = d.department_id);
![image-20221020204136437](https://i0.hdslb.com/bfs/album/383b518f00f5d1e37de32d4ed1fe2d925bb9b051.png)
4.4 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);
4.5 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);
5. 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。
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_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
-- 方式1
SELECT last_name,job_id,salary
FROM employees
WHERE salary > ( -- 单行子查询
SELECT MAX(salary)
FROM employees
WHERE job_id='SA_MAN'
);
-- 方式2
SELECT last_name,job_id,salary
FROM employees
WHERE salary > ALL ( -- 多行子查询
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,department_id
FROM employees
WHERE department_id IN(
SELECT DISTINCT department_id
FROM employees
WHERE last_name LIKE '%u%'
-- WHERE last_name REGEXP '[u]' --正则表达式
);
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
-- 方式1 多表连接 join on
SELECT worker.last_name,worker.salary
FROM employees worker
JOIN employees mar
ON worker.manager_id = mar.employee_id
WHERE mar.last_name='King';
-- 方式2 多行子查询
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(dept_avgsal)
FROM (
SELECT AVG(salary) dept_avgsal
FROM employees
GROUP BY department_id
) avg_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) AS avg_sal
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) AS avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal
LIMIT 0, 1
)
)
-- 方式四:
SELECT d.*
FROM departments d, (
SELECT department_id, AVG(salary) AS avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal
LIMIT 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)
-- 方式一:
SELECT d.*, (
SELECT AVG(salary)
FROM employees
WHERE department_id = d.department_id
) AS avg_sal
FROM departments d
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (
SELECT MIN(dept_avgsal)
FROM (
SELECT AVG(salary) AS dept_avgsal
FROM employees
GROUP BY department_id
) avg_sal
)
);
-- 方式二:
SELECT d.*
, (
SELECT AVG(salary)
FROM employees
WHERE department_id = d.`department_id`
) AS avg_sal
FROM departments d
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) <= ALL (
SELECT AVG(salary) AS avg_sal
FROM employees
GROUP BY department_id
)
);
-- 方式三:
SELECT d.*
, (
SELECT AVG(salary)
FROM employees
WHERE department_id = d.department_id
) AS avg_sal
FROM departments d
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (
SELECT AVG(salary) AS avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal
LIMIT 0, 1
)
)
-- 方式四:
SELECT d.*, dept_avg_sal.avg_sal
FROM departments d, (
SELECT department_id, AVG(salary) AS avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal
LIMIT 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
-- 方式一:
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) AS avg_sal
FROM employees
GROUP BY job_id
) job_avgsal
)
);
-- 方式二:
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
)
);
-- 方式三:
SELECT *
FROM jobs
WHERE job_id = (
SELECT job_id
FROM employees
GROUP BY job_id
HAVING AVG(salary) = (
SELECT AVG(salary) AS avg_sal
FROM employees
GROUP BY job_id
ORDER BY avg_sal DESC
LIMIT 0, 1
)
);
-- 方式四:
SELECT j.*
FROM jobs j, (
SELECT job_id, AVG(salary) AS avg_sal
FROM employees
GROUP BY job_id
ORDER BY avg_sal DESC
LIMIT 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 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:
SELECT employee_id, last_name, salary
FROM employees
WHERE employee_id IN (
SELECT DISTINCT manager_id
FROM employees
);
-- 方式2:
SELECT DISTINCT e1.employee_id, e1.last_name, e1.salary
FROM employees e1
JOIN employees e2
WHERE e1.employee_id = e2.manager_id;
-- 方式3:
SELECT employee_id, last_name, salary
FROM employees e1
WHERE EXISTS (
SELECT *
FROM employees e2
WHERE e2.manager_id = e1.employee_id
);
13.各个部门的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) AS max_sal
FROM employees
GROUP BY department_id
) dept_max_sal
)
);
-- 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) AS max_sal
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) AS max_sal
FROM employees
GROUP BY department_id
ORDER BY max_sal
LIMIT 0, 1
)
)
-- 方式4:
SELECT employee_id, MIN(salary)
FROM employees e, (
SELECT department_id, MAX(salary) AS max_sal
FROM employees
GROUP BY department_id
ORDER BY max_sal
LIMIT 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
-- 方式一:
SELECT employee_id, 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) AS avg_sal
FROM employees
GROUP BY department_id
) dept_sal
)
)
);
-- 方式二:
SELECT employee_id, 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 e
GROUP BY department_id
HAVING AVG(salary) >= ALL (
SELECT AVG(salary)
FROM employees
GROUP BY department_id
)
)
);
-- 方式三:
SELECT *
FROM employees
WHERE employee_id IN (
SELECT DISTINCT manager_id
FROM employees e, (
SELECT department_id, AVG(salary) AS avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal DESC
LIMIT 0, 1
) dept_avg_sal
WHERE e.department_id = 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
-- 方法一:
SELECT department_id
FROM departments d
WHERE department_id NOT IN (
SELECT DISTINCT department_id
FROM employees
WHERE job_id = 'ST_CLERK'
);
-- 方法二:
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
SELECT last_name
FROM employees e1
WHERE NOT EXISTS (
SELECT *
FROM employees e2
WHERE 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'
-- 方式1:
SELECT employee_id, last_name, hire_date, salary
FROM employees
WHERE manager_id = (
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 e2.`employee_id` = e1.manager_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)
-- 方式一:
SELECT employee_id, last_name, salary
FROM employees e1
WHERE salary > (
SELECT AVG(salary)
FROM employees e2
WHERE e2.department_id = e1.`department_id`
);
-- 方式二:
SELECT employee_id, last_name, salary
FROM employees e1, (
SELECT department_id, AVG(salary) AS avg_sal
FROM 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)
-- 方式1
SELECT department_name
FROM departments d
WHERE 5 < (
SELECT COUNT(*)
FROM employees e
WHERE d.department_id = e.department_id
);
-- 方式2
SELECT department_name
FROM departments
WHERE department_id IN (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING COUNT(*) > 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 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。