Python进阶----多表查询(内连,左连,右连), 子查询(in,带比较运算符)
Python进阶----多表查询(内连,左连,右连), 子查询(in,带比较运算符)
一丶多表查询
多表连接查询的应用场景:
连接是关系数据库模型的主要特点,也是区别于其他类型数据管理系的一个标志.
通常来说表与表之间的关系不必确定,也就时实体与实体之间的关系不紧密,检索数据时,使用连表操作增强灵活性.可以再连表查询时增加新的字段,为不同实体创建新的表.
多表联查的基本语句:
# 多表连接查询语法
select 字段
from 表1
INNER|LEFT|RIGHT join 表2
on 表1.字段=表2.字段;
# 条件过滤
where 条件
# 1.形成新的表
# 2.灵活的操控两个表的所有字段
# 3.提高效率
交叉连接:
即笛卡尔积,将两个表所有的记录进行配对,数据大量冗余,没有实际意义
### 查询 employee 员工表 和 department 部门表的笛卡尔积
# 数据大量冗余 ,没有实际意义
select * from employee,department;
+----+------------+--------+------+--------+------+--------------+
| id | name | sex | age | dep_id | id | name |
+----+------------+--------+------+--------+------+--------------+
| 1 | egon | male | 18 | 200 | 200 | 技术 |
| 1 | egon | male | 18 | 200 | 201 | 人力资源 |
| 1 | egon | male | 18 | 200 | 202 | 销售 |
| 1 | egon | male | 18 | 200 | 203 | 运营 |
| 2 | alex | female | 48 | 201 | 200 | 技术 |
| 2 | alex | female | 48 | 201 | 201 | 人力资源 |
| 2 | alex | female | 48 | 201 | 202 | 销售 |
| 2 | alex | female | 48 | 201 | 203 | 运营 |
| 3 | wupeiqi | male | 38 | 201 | 200 | 技术 |
| 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 |
| 3 | wupeiqi | male | 38 | 201 | 202 | 销售 |
| 3 | wupeiqi | male | 38 | 201 | 203 | 运营 |
| 4 | yuanhao | female | 28 | 202 | 200 | 技术 |
| 4 | yuanhao | female | 28 | 202 | 201 | 人力资源 |
| 4 | yuanhao | female | 28 | 202 | 202 | 销售 |
| 4 | yuanhao | female | 28 | 202 | 203 | 运营 |
| 5 | liwenzhou | male | 18 | 200 | 200 | 技术 |
| 5 | liwenzhou | male | 18 | 200 | 201 | 人力资源 |
| 5 | liwenzhou | male | 18 | 200 | 202 | 销售 |
| 5 | liwenzhou | male | 18 | 200 | 203 | 运营 |
| 6 | jingliyang | female | 18 | 204 | 200 | 技术 |
| 6 | jingliyang | female | 18 | 204 | 201 | 人力资源 |
| 6 | jingliyang | female | 18 | 204 | 202 | 销售 |
| 6 | jingliyang | female | 18 | 204 | 203 | 运营 |
+----+------------+--------+------+--------+------+--------------+
内连接:
内连接是一种最常用的连接类型,两个表的字段满足的连接条件,只显示两张表中互相匹配的项,其他不匹配项的不显示.
### 将员工表(employee)与部门表(department)进行连接, 员工表通过字段dep_id(部门id) 与 部门表字段id(部门id)进行连接
# 方式一 自连接 (特殊的内连接) 通过where 方式进行连接
select * from employee e ,department d where e.dep_id=d.id ;
# 结果
+----+-----------+--------+------+--------+------+--------------+
| id | name | sex | age | dep_id | id | name |
+----+-----------+--------+------+--------+------+--------------+
| 1 | egon | male | 18 | 200 | 200 | 技术 |
| 2 | alex | female | 48 | 201 | 201 | 人力资源 |
| 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 |
| 4 | yuanhao | female | 28 | 202 | 202 | 销售 |
| 5 | liwenzhou | male | 18 | 200 | 200 | 技术 |
+----+-----------+--------+------+--------+------+--------------+
# 方式二 通过内连接 ... inner join ... on方式连接
select * from employee e inner join department d on e.dep_id=d.id ;
# 结果
+----+-----------+--------+------+--------+------+--------------+
| id | name | sex | age | dep_id | id | name |
+----+-----------+--------+------+--------+------+--------------+
| 1 | egon | male | 18 | 200 | 200 | 技术 |
| 2 | alex | female | 48 | 201 | 201 | 人力资源 |
| 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 |
| 4 | yuanhao | female | 28 | 202 | 202 | 销售 |
| 5 | liwenzhou | male | 18 | 200 | 200 | 技术 |
+----+-----------+--------+------+--------+------+--------------+
外连接:
左外连接:
语法:A LEFT join B on A.XX=B.XX;
左表存显示所有,右表没有与左表匹配的则为 null.
右外连接:
语法:A RIGHT join B on A.XX=B.XX;
右表存显示所有,左表没有与右表匹配的则为 null.
### 员工表和部门表
# 左外连接 : 与左表没有匹配的则显示空,左表完全显示
select * from employee e left join department d on e.dep_id=d.id;
# 结果:
+----+------------+--------+------+--------+------+--------------+
| id | name | sex | age | dep_id | id | name |
+----+------------+--------+------+--------+------+--------------+
| 1 | egon | male | 18 | 200 | 200 | 技术 |
| 5 | liwenzhou | male | 18 | 200 | 200 | 技术 |
| 2 | alex | female | 48 | 201 | 201 | 人力资源 |
| 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 |
| 4 | yuanhao | female | 28 | 202 | 202 | 销售 |
| 6 | jingliyang | female | 18 | 204 | NULL | NULL | #### 重点~~~
+----+------------+--------+------+--------+------+--------------+
# 右外连接 : 与右表没有匹配的则显示空,右表完全显示
select * from employee e right join department d on e.dep_id=d.id;
# 结果:
+------+-----------+--------+------+--------+------+--------------+
| id | name | sex | age | dep_id | id | name |
+------+-----------+--------+------+--------+------+--------------+
| 1 | egon | male | 18 | 200 | 200 | 技术 |
| 2 | alex | female | 48 | 201 | 201 | 人力资源 |
| 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 |
| 4 | yuanhao | female | 28 | 202 | 202 | 销售 |
| 5 | liwenzhou | male | 18 | 200 | 200 | 技术 |
| NULL | NULL | NULL | NULL | NULL | 203 | 运营 | #### 重点~~
+------+-----------+--------+------+--------+------+--------------+
全连接:
union 关键字
mysql 不支持全外连接 full JOIN
mysql 使用 union 连接左连接和右连接，得到全连接
### union 全连接
# 必须是 左连接 + 右连接
select * from employee e left join department d on e.dep_id=d.id
union
select * from employee e right join department d on e.dep_id=d.id;
# 结果:
+------+------------+--------+------+--------+------+--------------+
| id | name | sex | age | dep_id | id | name |
+------+------------+--------+------+--------+------+--------------+
| 1 | egon | male | 18 | 200 | 200 | 技术 |
| 5 | liwenzhou | male | 18 | 200 | 200 | 技术 |
| 2 | alex | female | 48 | 201 | 201 | 人力资源 |
| 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 |
| 4 | yuanhao | female | 28 | 202 | 202 | 销售 |
| 6 | jingliyang | female | 18 | 204 | NULL | NULL | ### 重点 👈 ~~~
| NULL | NULL | NULL | NULL | NULL | 203 | 运营 | ### 重点 👈 ~~~
+------+------------+--------+------+--------+------+--------------+
### 全连接 错误用法
# 1. select * from employee e left join department d on e.dep_id=d.id 可以得到 左连接
# 2. select * from department d left join employee e on e.dep_id=d.id 通过调换表的顺序可以得到和右连接一样的效果
# 3. 但是 这样组合起来的东西并不是全连接
select * from employee e left join department d on e.dep_id=d.id union select * from department d left join employee e on e.dep_id=d.id ;
# 结果:👇 这并不是全连接
+------+--------------+--------+-----------+--------+------+--------------+
| id | name | sex | age | dep_id | id | name |
+------+--------------+--------+-----------+--------+------+--------------+
| 1 | egon | male | 18 | 200 | 200 | 技术 |
| 5 | liwenzhou | male | 18 | 200 | 200 | 技术 |
| 2 | alex | female | 48 | 201 | 201 | 人力资源 |
| 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 |
| 4 | yuanhao | female | 28 | 202 | 202 | 销售 |
| 6 | jingliyang | female | 18 | 204 | NULL | NULL |
| 200 | 技术 | 1 | egon | male | 18 | 200 |
| 201 | 人力资源 | 2 | alex | female | 48 | 201 |
| 201 | 人力资源 | 3 | wupeiqi | male | 38 | 201 |
| 202 | 销售 | 4 | yuanhao | female | 28 | 202 |
| 200 | 技术 | 5 | liwenzhou | male | 18 | 200 |
| 203 | 运营 | NULL | NULL | NULL | NULL | NULL |
+------+--------------+--------+-----------+--------+------+--------------+
准备employee 和 department 表:
#建表
create table department(
id int,
name varchar(20)
);
create table employee(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') not null default 'male',
age int,
dep_id int
);
#插入数据
insert into department values
(200,'技术'),
(201,'人力资源'),
(202,'销售'),
(203,'运营');
insert into employee(name,sex,age,dep_id) values
('egon','male',18,200),
('alex','female',48,201),
('wupeiqi','male',38,201),
('yuanhao','female',28,202),
('liwenzhou','male',18,200),
('jingliyang','female',18,204)
;
练习题:
# 1. 以 内连接的方式 查询employee和department表，并且employee表中的age字段值必须大于25,即找出年龄大于25岁的员工以及员工所在的部门
select e.name,d.name from employee e inner join department d on e.dep_id=d.id where e.age>25;
# 2. 以 内连接的方式 查询employee和department表，年龄大于25 ,并且以age字段的升序方式显示
select * from employee e inner join department d on e.dep_id=d.id where e.age>25 order by e.age;
# 3. 使用 自连接方式 查询employee和department表，年龄大于25 ,并且以age字段的升序方式显示
select * from employee e , department d where e.dep_id=d.id and e.age>25 order by e.age;
二丶子查询
什么是子查询:
1.子查询是将一个查询语句嵌套另一个语句中
2.内层查询语句的结果,可以作为外层查询的条件(where)
3.子查询中可以包含:IN , NOT , ANY , ALL , EXISTS 和 NOT EXISTS等关键字
4.可以包含比较运算符: = , != , > , < 等
5.通俗:携带 两个select 就是子查询.
6.子查询也可以作为字段(只能是单个值)
带IN关键字的子查询:
# 1. 查询平均年龄在25岁以上的部门名
select * from department where id in (select dep_id from employee group by dep_id having avg(age));
# 2. 查看 '技术' 部员工姓名
select name from employee where dep_id=(select id from department where name='技术');
# 3.查看不足1人的部门名(子查询得到的是有人的部门id)
select name from department where id not in(select dep_id from employee );
带比较运算符的子查询:
# 1. 查询大于所有人平均年龄的员工名与年龄
select name,age from employee where age>(select avg(age) from employee);
# 2. 查询大于部门内平均年龄的员工名、年龄
select e.name,e.age from employee e inner join (select dep_id,avg(age) as avg_age from employee group by dep_id) as t on e.dep_id =t.dep_id where e.age >avg_age ;
带exists关键字的子查询:
exists表示:内层查询语句返回的是一个bool值,True时执行外层查询,False时不执行外层查询
# 1 . department表中存在dept_id=203，执行查询员工表
select * from employee
where exists (select * from department where id=203);
# 结果:👇
mysql> select * from employee
-> where exists (select * from department where id=203);
+----+------------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+------------+--------+------+--------+
| 1 | egon | male | 18 | 200 |
| 2 | alex | female | 48 | 201 |
| 3 | wupeiqi | male | 38 | 201 |
| 4 | yuanhao | female | 28 | 202 |
| 5 | liwenzhou | male | 18 | 200 |
| 6 | jingliyang | female | 18 | 204 |
+----+------------+--------+------+--------+
6 rows in set (0.00 sec)
# 2.department表中存在dept_id=205，执行查询员工表
select * from employee
where exists (select * from department where id=205);
# 结果:👇
mysql> select * from employee
-> where exists (select * from department where id=205);
Empty set (0.00 sec)
select 字段 子查询 (效率极低,不推荐使用,了解):
1.字段可以使一个子查询,但是这个字段必须是唯一值,否则报错
2.子查询处理可以放在条件中,还可以放在连表中,还可以放在select字段(要求查询的结果必须是一个单行单列的值)中.
## 准备 emp 表数据
create table emp(
id int not null unique auto_increment,
name varchar(20) not null,
sex enum('male','female') not null default 'male', #大部分是男的
age int(3) unsigned not null default 28,
hire_date date not null,
post varchar(50),
post_comment varchar(100),
salary double(15,2),
office int, #一个部门一个屋子
depart_id int
);
insert into emp(name,sex,age,hire_date,post,salary,office,depart_id) values
('egon','male',18,'20170301','老男孩驻沙河办事处外交大使',7300.33,401,1), #以下是教学部
('alex','male',78,'20150302','teacher',1000000.31,401,1),
('wupeiqi','male',81,'20130305','teacher',8300,401,1),
('yuanhao','male',73,'20140701','teacher',3500,401,1),
('liwenzhou','male',28,'20121101','teacher',2100,401,1),
('jingliyang','female',18,'20110211','teacher',9000,401,1),
('jinxin','male',18,'19000301','teacher',30000,401,1),
('成龙','male',48,'20101111','teacher',10000,401,1),
('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是销售部门
('丫丫','female',38,'20101101','sale',2000.35,402,2),
('丁丁','female',18,'20110312','sale',1000.37,402,2),
('星星','female',18,'20160513','sale',3000.29,402,2),
('格格','female',28,'20170127','sale',4000.33,402,2),
('张野','male',28,'20160311','operation',10000.13,403,3), #以下是运营部门
('程咬金','male',18,'19970312','operation',20000,403,3),
('程咬银','female',18,'20130311','operation',19000,403,3),
('程咬铜','male',18,'20150411','operation',18000,403,3),
('程咬铁','female',18,'20140512','operation',17000,403,3)
;
示例:select A表name ,B表age(通过关联A表和B表的name字段) from A表
# 查询 emp 表中的name 字段 和 employee 表中的age字段, 两个表的的name字段进行关联
select name as n ,(select age from employee as e where e.name=n ) as employee_Age from emp;
# 分析: n字段来自表emp , employee_Age字段来自 employee 字段
+------------+--------------+
| n | employee_Age |
+------------+--------------+
| egon | 18 |
| alex | 48 |
| wupeiqi | 38 |
| yuanhao | 28 |
| liwenzhou | 18 |
| jingliyang | 18 |
| jinxin | NULL |
| 成龙 | NULL |
| 歪歪 | NULL |
| 丫丫 | NULL |
| 丁丁 | NULL |
| 星星 | NULL |
| 格格 | NULL |
| 张野 | NULL |
| 程咬金 | NULL |
| 程咬银 | NULL |
| 程咬铜 | NULL |
| 程咬铁 | NULL |
+------------+--------------+
