子查询
- 1. 由一个具体的需求,引入子查询
- 2. 称谓的规范:外查询(或主查询)、内查询(或子查询)
- 3. 子查询的分类
- 4. 单行子查询
- 4.1 单行操作符: = != > >= < <=
- 4.2 子查询中的空值问题
- 4.3 非法使用子查询
- 5.多行子查询
- 5.1 多行子查询的操作符: IN ANY ALL SOME(同ANY)
- 5.2举例:
- 5.3 空值问题(存在记录行为null,会导致没有结果)
- 6. 相关子查询
- 6.1 子查询可以出现的位置
- 6.2 EXISTS 与 NOT EXISTS关键字
- 子查询的练习
- 1.查询和Zlotkey相同部门的员工姓名和工资
- 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。
- 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
- 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
- 5.查询在部门的location_id为1700的部门工作的员工的员工号
- 6.查询管理者是King的员工姓名和工资
- 7.查询工资最低的员工信息: last_name, salary
- 8.查询平均工资最低的部门信息
- 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)
- 10.查询平均工资最高的 job 信息
- 11.查询平均工资高于公司平均工资的部门有哪些?
- 12.查询出公司中所有 manager 的详细信息
- 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?
- 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
- 15. 查询部门的部门号,其中不包括job_id是"ST_CLERK"的部门号
- 16. 选择所有没有管理者的员工的last_name
- 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'
- 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)
- 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)
- 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)
1. 由一个具体的需求,引入子查询
需求:谁的工资比Abel的高?
#方式1:
SELECT salary
FROM employees
WHERE last_name = 'Abel';SELECT last_name,salary
FROM employees
WHERE salary > 11000;#方式2:自连接
SELECT e2.last_name,e2.salary
FROM employees e1,employees e2
WHERE e2.`salary` > e1.`salary` #多表的连接条件
AND e1.last_name = 'Abel';#方式3:子查询
SELECT last_name,salary
FROM employees
WHERE salary > (SELECT salaryFROM employeesWHERE last_name = 'Abel');
2. 称谓的规范:外查询(或主查询)、内查询(或子查询)
- 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
不推荐:
SELECT last_name,salary
FROM employees
WHERE (SELECT salaryFROM employeesWHERE last_name = 'Abel') < salary;
3. 子查询的分类
角度1:从内查询返回的结果的条目数
单行子查询 vs 多行子查询
角度2:内查询是否被执行多次
相关子查询 vs 不相关子查询
比如:相关子查询的需求:查询工资大于本部门平均工资的员工信息。
不相关子查询的需求:查询工资大于本公司平均工资的员工信息。
子查询的编写技巧(或步骤):① 从里往外写 ② 从外往里写
4. 单行子查询
4.1 单行操作符: = != > >= < <=
题目: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 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 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号员工的manager_id和department_id相同的其他员工的employee_id,manager_id,department_id。
#方式1:
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;#方式2:了解
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;
题目:查询最低工资大于110号部门最低工资的部门id和其最低工资
SELECT department_id,MIN(salary)
FROM employees
WHERE department_id IS NOT NULL
GROUP BY department_id
HAVING MIN(salary) > (SELECT MIN(salary)FROM employeesWHERE department_id = 110);
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;
4.2 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');
4.3 非法使用子查询
错误:Subquery returns more than 1 row
– 子查询有多个结果
SELECT employee_id, last_name
FROM employees
WHERE salary =(SELECT MIN(salary)FROM employeesGROUP BY department_id);
5.多行子查询
5.1 多行子查询的操作符: IN ANY ALL SOME(同ANY)
5.2举例:
IN
SELECT employee_id, last_name
FROM employees
WHERE salary IN(SELECT MIN(salary)FROM employeesGROUP BY department_id);
ANY / ALL
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 job_id <> 'IT_PROG'
AND salary < ANY (SELECT salaryFROM employeesWHERE 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 job_id <> 'IT_PROG'
AND salary < ALL (SELECT salaryFROM employeesWHERE job_id = 'IT_PROG');
题目:查询平均工资最低的部门id
MySQL中聚合函数是不能嵌套使用的。
#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) t_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)
5.3 空值问题(存在记录行为null,会导致没有结果)
SELECT last_name
FROM employees
WHERE employee_id NOT IN (-- 改为 in 的时候,由于employee_id是主键,所以不会为null,所以没有报错SELECT manager_idFROM employees);
6. 相关子查询
回顾:查询员工中工资大于公司平均工资的员工的last_name,salary和其department_id
6.1 子查询可以出现的位置
SELECT last_name,salary,department_id
FROM employees
WHERE salary > (SELECT AVG(salary)FROM employees);
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1:使用相关子查询,where中子查询
SELECT last_name,salary,department_id
FROM employees e1
WHERE salary > (SELECT AVG(salary)FROM employees e2WHERE department_id = e1.`department_id`);#方式2:在FROM中声明子查询,采用临时表
SELECT e.last_name,e.salary,e.department_id
FROM employees e,(SELECT department_id,AVG(salary) avg_salFROM employeesGROUP 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
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`) ASC;
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 ...,....
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 *
FROM job_history;SELECT employee_id,last_name,job_id
FROM employees e
WHERE 2 <= (SELECT COUNT(*)FROM job_history jWHERE e.`employee_id` = j.`employee_id`)
6.2 EXISTS 与 NOT EXISTS关键字
题目:查询公司管理者的employee_id,last_name,job_id,department_id信息
#方式1:自连接
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_idFROM employees);#方式3:使用EXISTS
SELECT employee_id,last_name,job_id,department_id
FROM employees e1
WHERE EXISTS (-- 查到为止,没查到,会继续往下面查SELECT *FROM employees e2WHERE e1.`employee_id` = e2.`manager_id`);
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1:
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;
-- or 下面的更容易理解
SELECT d.department_id,d.department_name
FROM departments d left JOIN employees e
ON e.`department_id` = d.`department_id`
WHERE e.`department_id` IS NULL;#方式2:
SELECT department_id,department_name
FROM departments d
WHERE NOT EXISTS (SELECT *FROM employees eWHERE d.`department_id` = e.`department_id`);SELECT COUNT(*)
FROM departments;
子查询的练习
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_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 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 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 employee_id,last_name
FROM employees
WHERE department_id IN (SELECT DISTINCT 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,manager_id
FROM employees
WHERE manager_id IN (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
#方式1:
SELECT *
FROM departments
WHERE department_id = (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary ) = (SELECT MIN(avg_sal)FROM (SELECT AVG(salary) avg_salFROM employeesGROUP BY department_id) t_dept_avg_sal));
#方式2:
SELECT *
FROM departments
WHERE department_id = (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary ) <= ALL(SELECT AVG(salary)FROM employeesGROUP BY department_id));#方式3: LIMIT
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_sal ASCLIMIT 1 ));#方式4:
SELECT d.*
FROM departments d,(SELECT department_id,AVG(salary) avg_salFROM employeesGROUP BY department_idORDER BY avg_sal ASCLIMIT 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_idFROM employeesGROUP BY department_idHAVING AVG(salary ) = (SELECT MIN(avg_sal)FROM (SELECT AVG(salary) avg_salFROM employeesGROUP 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_idFROM employeesGROUP BY department_idHAVING AVG(salary ) <= ALL(SELECT AVG(salary)FROM employeesGROUP 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_idFROM employeesGROUP BY department_idHAVING AVG(salary ) =(SELECT AVG(salary) avg_salFROM employeesGROUP BY department_idORDER BY avg_sal ASCLIMIT 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_salFROM employeesGROUP BY department_idORDER BY avg_sal ASCLIMIT 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_idFROM employeesGROUP BY job_idHAVING AVG(salary) = (SELECT MAX(avg_sal)FROM (SELECT AVG(salary) avg_salFROM employeesGROUP BY job_id) t_job_avg_sal));#方式2:
SELECT *
FROM jobs
WHERE job_id = (SELECT job_idFROM employeesGROUP BY job_idHAVING AVG(salary) >= ALL(SELECT AVG(salary) FROM employeesGROUP BY job_id));#方式3:
SELECT *
FROM jobs
WHERE job_id = (SELECT job_idFROM employeesGROUP BY job_idHAVING AVG(salary) =(SELECT AVG(salary) avg_salFROM employeesGROUP BY job_idORDER BY avg_sal DESCLIMIT 0,1));#方式4:
SELECT j.*
FROM jobs j,(SELECT job_id,AVG(salary) avg_salFROM employeesGROUP BY job_idORDER BY avg_sal DESCLIMIT 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_idFROM employees);#方式3:使用EXISTS
SELECT employee_id,last_name,job_id,department_id
FROM employees e1
WHERE EXISTS (SELECT *FROM employees e2WHERE 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_idFROM employeesGROUP BY department_idHAVING MAX(salary) = (SELECT MIN(max_sal)FROM (SELECT MAX(salary) max_salFROM employeesGROUP 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_idFROM employeesGROUP BY department_idHAVING MAX(salary) <= ALL (SELECT MAX(salary)FROM employeesGROUP BY department_id));#方式3:
SELECT MIN(salary)
FROM employees
WHERE department_id = (SELECT department_idFROM employeesGROUP BY department_idHAVING MAX(salary) = (SELECT MAX(salary) max_salFROM employeesGROUP BY department_idORDER BY max_sal ASCLIMIT 0,1));#方式4:
SELECT MIN(salary)
FROM employees e,(SELECT department_id,MAX(salary) max_salFROM employeesGROUP BY department_idORDER BY max_sal ASCLIMIT 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_idFROM employeesWHERE department_id = (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) = (SELECT MAX(avg_sal)FROM (SELECT AVG(salary) avg_salFROM employeesGROUP 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_idFROM employeesWHERE department_id = (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) >= ALL (SELECT AVG(salary) avg_salFROM employeesGROUP BY department_id)));#方式3:
SELECT last_name, department_id, email, salary
FROM employees
WHERE employee_id IN (SELECT DISTINCT manager_idFROM employees e,(SELECT department_id,AVG(salary) avg_salFROM employeesGROUP BY department_idORDER BY avg_sal DESCLIMIT 0,1) t_dept_avg_salWHERE 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_idFROM employeesWHERE job_id = 'ST_CLERK');#方式2:
SELECT department_id
FROM departments d
WHERE NOT EXISTS (SELECT *FROM employees eWHERE 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 mgrWHERE 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_idFROM employeesWHERE last_name = 'De Haan');#方式2:
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)
#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 e2WHERE 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_salFROM employeesGROUP 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 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 * FROM locations;SELECT country_id
FROM locations l
WHERE 2 < (SELECT COUNT(*)FROM departments dWHERE 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。