MYSQL 第六课分组查询连接查询

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MYSQL 第六课分组查询连接查询

2023-12-07 01:08| 来源: 网络整理| 查看: 265

分组查询

/*语法：

select 查询列表from 表【where 筛选条件】group by 分组的字段（分成若干组）【order by 排序的字段】;

特点：1、和分组函数一同查询的字段必须是group by后出现的字段2、筛选分为两类：分组前筛选和分组后筛选针对的表位置连接的关键字分组前筛选原始表 group by前 where

分组后筛选 group by后的结果集 group by后 having

问题1：分组函数做筛选能不能放在where后面答：不能

问题2：where——group by——having

一般来讲，能用分组前筛选的，尽量使用分组前筛选，提高效率

3、分组可以按单个字段也可以按多个字段（多个字段间用逗号隔开，没有顺序要求）4、可以搭配着排序使用

#引入：查询每个部门的员工个数

SELECT COUNT(*) FROM employees WHERE department_id=90; #1.简单的分组

#案例1：查询每个工种的员工平均工资

SELECT AVG(salary),job_id FROM employees GROUP BY job_id;

#案例2：查询每个位置的部门个数

SELECT COUNT(*),location_id FROM departments GROUP BY location_id; #2、可以实现分组前的筛选

#案例1：查询邮箱中包含a字符的每个部门的最高工资

SELECT MAX(salary),department_id,email FROM employees WHERE email LIKE '%a%' GROUP BY department_id;

#案例2：查询有奖金的每个领导手下员工的平均工资

SELECT AVG(salary),manager_id FROM employees WHERE commission_pct IS NOT NULL GROUP BY manager_id;

#3、分组后筛选

#案例：查询哪个部门的员工个数>5

#①查询每个部门的员工个数

SELECT COUNT(*),department_id FROM employees GROUP BY department_id;

#② 筛选刚才①结果

SELECT COUNT(*),department_id FROM employees GROUP BY department_id HAVING COUNT(*)>5;

#案例2：每个工种有奖金的员工的最高工资>10000的工种编号和最高工资

SELECT job_id,MAX(salary) FROM employees WHERE commission_pct IS NOT NULL GROUP BY job_id HAVING MAX(salary)>10000;

#案例3：领导编号>102的每个领导手下的最低工资大于5000的领导编号和最低工资

# 1 查询每个领导手下的最低工资# 2 领导编号>102# 3 最低工资大于5000

SELECT manager_id,MIN(salary) FROM employees WHERE manager_id>102 GROUP BY manager_id HAVING MIN(salary)>5000;

#4.添加排序

#案例：每个工种有奖金的员工的最高工资>6000的工种编号和最高工资,按最高工资升序

SELECT job_id,MAX(salary) m FROM employees WHERE commission_pct IS NOT NULL GROUP BY job_id HAVING m>6000 ORDER BY m ;

#5.按多个字段分组

#案例：查询每个工种每个部门的最低工资,并按最低工资降序

SELECT MIN(salary),job_id,department_id FROM employees GROUP BY department_id,job_id ORDER BY MIN(salary) DESC;

#6 按表达式或函数分组 #案例按员工姓名的长度分组，查询每一组的员工个数，筛选员工个数大于5 的有哪些 SELECT COUNT(*),LENGTH(last_name) FROM employees GROUP BY LENGTH(last_name) HAVING COUNT(*)>5;

例题

#1.查询各job_id的员工工资的最大值，最小值，平均值，总和，并按job_id升序

SELECT MAX(salary),MIN(salary),AVG(salary),SUM(salary),job_id FROM employees GROUP BY job_id ORDER BY job_id;

#2.查询员工最高工资和最低工资的差距（DIFFERENCE）

SELECT MAX(salary)-MIN(salary) DIFFRENCE FROM employees;

#3.查询各个管理者手下员工的最低工资，其中最低工资不能低于6000，没有管理者的员工不计算在内

SELECT MIN(salary),manager_id FROM employees WHERE manager_id IS NOT NULL GROUP BY manager_id HAVING MIN(salary)>=6000;

#4.查询所有部门的编号，员工数量和工资平均值,并按平均工资降序

SELECT department_id,COUNT(*),AVG(salary) a FROM employees GROUP BY department_id ORDER BY a DESC;

#5.选择具有各个job_id的员工人数

SELECT COUNT(*) 个数,job_id FROM employees GROUP BY job_id;

连接查询

含义：又称多表查询，当查询的字段来自于多个表时，就会用到连接查询

笛卡尔乘积现象：表1 有m行，表2有n行，结果=m*n行

发生原因：没有有效的连接条件如何避免：添加有效的连接条件

分类：按年代分类：

sql92标准:仅仅支持内连接 sql99标准【推荐】：支持内连接+外连接（左外和右外）+交叉连接

按功能分类：内连接：等值连接非等值连接自连接外连接：左外连接右外连接全外连接

交叉连接

SELECT * FROM beauty;

SELECT * FROM boys;

SELECT NAME,boyName FROM boys,beautyWHERE beauty.boyfriend_id= boys.id;

#一、sql92标准# 1、等值连接

① 多表等值连接的结果为多表的交集部分②n表连接，至少需要n-1个连接条件③ 多表的顺序没有要求④一般需要为表起别名⑤可以搭配前面介绍的所有子句使用，比如排序、分组、筛选

#1、基础

#案例1：查询女神名和对应的男神名

SELECT NAME,boyName FROM boys,beauty WHERE beauty.boyfriend_id= boys.id;

#案例2：查询员工名和对应的部门名

SELECT last_name,department_name FROM employees,departments WHERE employees.`department_id`=departments.`department_id`;

#2、为表起别名

/*①提高语句的简洁度②区分多个重名的字段

注意：如果为表起了别名，则查询的字段就不能使用原来的表名去限定

*/#查询员工名、工种号、工种名

SELECT e.last_name,e.job_id,j.job_title FROM employees e,jobs j WHERE e.`job_id`=j.`job_id`; #3、两个表的顺序是否可以调换

#查询员工名、工种号、工种名

SELECT last_name,department_name,commission_pct FROM employees e,departments d WHERE e.`department_id`=d.`department_id` AND e.`commission_pct` IS NOT NULL;

#4、可以加筛选

#案例：查询有奖金的员工名、部门名

SELECT last_name,department_name,commission_pct FROM employees e,departments d WHERE e.`department_id`=d.`department_id` AND e.`commission_pct` IS NOT NULL;

#案例2：查询城市名中第二个字符为o的部门名和城市名

SELECT department_name,city FROM departments d,locations l WHERE d.`location_id` = l.`location_id` AND city LIKE '_o%';

#5、可以加分组

#案例1：查询每个城市的部门个数

SELECT COUNT(*) 个数,city FROM departments d,locations l WHERE d.`location_id`=l.`location_id` GROUP BY city;

#6、可以加排序

#案例：查询每个工种的工种名和员工的个数，并且按员工个数降序

SELECT job_title,COUNT(*) FROM employees e,jobs j WHERE e.`job_id`=j.`job_id` GROUP BY job_title ORDER BY COUNT(*) DESC; #7、可以实现三表连接？

#案例：查询员工名、部门名和所在的城市

SELECT last_name,department_name,city FROM employees e,departments d,locations l WHERE e.`department_id`=d.`department_id` AND d.`location_id`=l.`location_id` AND city LIKE 's%' ORDER BY department_name DESC; #2、非等值连接

#案例1：查询员工的工资和工资级别

SELECT salary,grade_level FROM employees e,job_grades g WHERE salary BETWEEN g.`lowest_sal` AND g.`highest_sal` AND g.`grade_level`='A';

/*select salary,employee_id from employees;

select * from job_grades;

CREATE TABLE job_grades(grade_level VARCHAR(3), lowest_sal int, highest_sal int);

INSERT INTO job_gradesVALUES ('A', 1000, 2999);

INSERT INTO job_gradesVALUES ('B', 3000, 5999);

INSERT INTO job_gradesVALUES('C', 6000, 9999);

INSERT INTO job_gradesVALUES('D', 10000, 14999);

INSERT INTO job_gradesVALUES('E', 15000, 24999);

INSERT INTO job_gradesVALUES('F', 25000, 40000);

#3、自连接

#案例：查询员工名和上级的名称 (找了两遍)

SELECT e.employee_id,e.last_name,m.employee_id,m.last_name FROM employees e,employees m WHERE e.`manager_id`=m.`employee_id`;

#1.显示所有员工的姓名，部门号和部门名称。

USE myemployees; SELECT last_name,d.department_id,department_name FROM employees e,departments d WHERE e.`department_id` = d.`department_id`;

#2.查询90号部门员工的job_id和90号部门的location_id

SELECT job_id,location_id FROM employees e,departments d WHERE e.`department_id`=d.`department_id` AND e.`department_id`=90;

#3. 选择所有有奖金的员工的last_name , department_name , location_id , city

SELECT last_name , department_name , l.location_id , city FROM employees e,departments d,locations l WHERE e.department_id = d.department_id AND d.location_id=l.location_id AND e.commission_pct IS NOT NULL;

#4.选择city在Toronto工作的员工的last_name , job_id , department_id , department_name

SELECT last_name , job_id , d.department_id , department_name FROM employees e,departments d ,locations l WHERE e.department_id = d.department_id AND d.location_id=l.location_id AND city = 'Toronto';

#5.查询每个工种、每个部门的部门名、工种名和最低工资

SELECT department_name,job_title,MIN(salary) 最低工资 FROM employees e,departments d,jobs j WHERE e.`department_id`=d.`department_id` AND e.`job_id`=j.`job_id` GROUP BY department_name,job_title;

#6.查询每个国家下的部门个数大于2的国家编号

SELECT country_id,COUNT(*) 部门个数 FROM departments d,locations l WHERE d.`location_id`=l.`location_id` GROUP BY country_id HAVING 部门个数>2;

#7、选择指定员工的姓名，员工号，以及他的管理者的姓名和员工号，结果类似于下面的格式employees Emp# manager Mgr#kochhar 101 king 100

SELECT e.last_name employees,e.employee_id "Emp#",m.last_name manager,m.employee_id "Mgr#" FROM employees e,employees m WHERE e.manager_id = m.employee_id AND e.last_name='kochhar';

#二、sql99语法

/*语法： select 查询列表 from 表1 别名【连接类型】 join 表2 别名 on 连接条件【where 筛选条件】【group by 分组】【having 筛选条件】【order by 排序列表】

分类：内连接（★）：inner外连接左外(★):left 【outer】右外(★)：right 【outer】全外：full【outer】交叉连接：cross

*/#一）内连接/*语法：

select 查询列表from 表1 别名inner join 表2 别名on 连接条件;

分类：等值非等值自连接

特点：①添加排序、分组、筛选②inner可以省略③ 筛选条件放在where后面，连接条件放在on后面，提高分离性，便于阅读④inner join连接和sql92语法中的等值连接效果是一样的，都是查询多表的交集

*/#1、等值连接

#案例1.查询员工名、部门名

SELECT last_name, department_name FROM departments d JOIN employees e ON e.`department_id` = d.`department_id` ;

#案例2.查询名字中包含e的员工名和工种名（添加筛选）

SELECT last_name,job_title FROM employees e INNER JOIN jobs j ON e.`job_id`= j.`job_id` WHERE e.`last_name` LIKE '%e%';

#3. 查询部门个数>3的城市名和部门个数，（添加分组+筛选）

#①查询每个城市的部门个数#②在①结果上筛选满足条件的

SELECT city,COUNT(*) 部门个数 FROM departments d INNER JOIN locations l ON d.`location_id`=l.`location_id` GROUP BY city HAVING COUNT(*)>3;

#案例4.查询哪个部门的员工个数>3的部门名和员工个数，并按个数降序（添加排序）

#①查询每个部门的员工个数

SELECT city,COUNT(*) 部门个数 FROM departments d INNER JOIN locations l ON d.`location_id`=l.`location_id` GROUP BY city HAVING COUNT(*)>3;

#② 在①结果上筛选员工个数>3的记录，并排序

SELECT COUNT(*) 个数,department_name FROM employees e INNER JOIN departments d ON e.`department_id`=d.`department_id` GROUP BY department_name HAVING COUNT(*)>3 ORDER BY COUNT(*) DESC;

#5.查询员工名、部门名、工种名，并按部门名降序（添加三表连接）

SELECT last_name,department_name,job_title FROM employees e INNER JOIN departments d ON e.`department_id`=d.`department_id` INNER JOIN jobs j ON e.`job_id` = j.`job_id` ORDER BY department_name DESC;

#二）非等值连接

#查询员工的工资级别

SELECT salary,grade_level FROM employees e JOIN job_grades g ON e.`salary` BETWEEN g.`lowest_sal` AND g.`highest_sal`;

#查询工资级别的个数>20的个数，并且按工资级别降序

SELECT COUNT(*),grade_level FROM employees e JOIN job_grades g ON e.`salary` BETWEEN g.`lowest_sal` AND g.`highest_sal` GROUP BY grade_level HAVING COUNT(*)>20 ORDER BY grade_level DESC;

#三）自连接

#查询员工的名字、上级的名字

SELECT e.last_name,m.last_name FROM employees e JOIN employees m ON e.`manager_id`= m.`employee_id`;

#查询姓名中包含字符k的员工的名字、上级的名字

SELECT e.last_name,m.last_name FROM employees e JOIN employees m ON e.`manager_id`= m.`employee_id` WHERE e.`last_name` LIKE '%k%';

#二、外连接

/* 应用场景：用于查询一个表中有，另一个表没有的记录

特点： 1、外连接的查询结果为主表中的所有记录如果从表中有和它匹配的，则显示匹配的值如果从表中没有和它匹配的，则显示null 外连接查询结果=内连接结果+主表中有而从表没有的记录 2、左外连接，left join左边的是主表右外连接，right join右边的是主表 3、左外和右外交换两个表的顺序，可以实现同样的效果 4、全外连接=内连接的结果+表1中有但表2没有的+表2中有但表1没有的 */ #引入：查询男朋友不在男神表的的女神名

SELECT * FROM beauty; SELECT * FROM boys;

#左外连接 SELECT b.*,bo.* FROM beauty b LEFT OUTER JOIN boys bo ON b.`boyfriend_id` = bo.`id`; #WHERE bo.`id` IS NULL; 右外连接 SELECT b.*,bo.* FROM boys bo RIGHT OUTER JOIN beauty b ON b.`boyfriend_id` = bo.`id` WHERE bo.`id` IS NULL;

#案例1：查询哪个部门没有员工 #左外

SELECT d.*,e.employee_id FROM departments d LEFT OUTER JOIN employees e ON d.`department_id` = e.`department_id` WHERE e.`employee_id` IS NULL;

#右外

SELECT d.*,e.employee_id FROM employees e RIGHT OUTER JOIN departments d ON d.`department_id` = e.`department_id` WHERE e.`employee_id` IS NULL; #全外 USE girls; SELECT b.*,bo.* FROM beauty b FULL OUTER JOIN boys bo ON b.`boyfriend_id` = bo.id;

#交叉连接

SELECT b.*,bo.* FROM beauty b CROSS JOIN boys bo;

#sql92和 sql99pk /* 功能：sql99支持的较多可读性：sql99实现连接条件和筛选条件的分离，可读性较高 */

#一、查询编号>3的女神的男朋友信息，如果有则列出详细，如果没有，用null填充

SELECT b.id,b.name,bo.* FROM beauty b LEFT OUTER JOIN boys bo ON b.`boyfriend_id` = bo.`id` WHERE b.`id`>3;

#二、查询哪个城市没有部门

SELECT city FROM departments d RIGHT OUTER JOIN locations l ON d.`location_id`=l.`location_id` WHERE d.`department_id` IS NULL;

#三、查询部门名为SAL或IT的员工信息

SELECT e.*,d.department_name,d.`department_id` FROM departments d LEFT JOIN employees e ON d.`department_id` = e.`department_id` WHERE d.`department_name` IN('SAL','IT'); SELECT * FROM departments WHERE `department_name` IN('SAL','IT');

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MYSQL 第六课分组查询连接查询

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