multi-table query

Classification of multi table query

1. Internal connection
2. External connection
3. Cross connect （ understand ）

notes ： The GUI manager used is ：Navicat Permium 12

Internal connection

Essence is ： Intersection of multiple tables .

#user_info surface 
create table user_info(
      id int(2) primary key,
      user_name varchar(12) unique,
      password varchar(15) not null,
      real_name varchar(8) not null,
      age int(3)
); 

#address surface 
create table address(
      id int(2) primary key,
      user_id int(2) not null,
      real_name varchar(8),
      mobile char(11),
      address varchar(150)
); 
insert into user_info values(1,' Shallow singing of happiness ','[email protected]',' Wang Xiaoming ',12);
insert into address values(1,1,' Xiao Ming Wang ','15516472282',' Taiyuan, Shanxi ');
insert into address values(2,1,' Wang Xin ','18404905139',' Datong, Shanxi ');
insert into address values(3,1,' Ren Jian ','15333021730',' Shanxi Jincheng ');
insert into user_info values(2,'ぅ man 's life is like a dream 〤','56701wz',' Wang Nan ',36);
insert into address values(4,2,' Wang Nan ','15010303314',' Beijing haidian ');
insert into address values(5,2,' Zhao Jie ','18435224278',' Shanxi changzhi ');
insert into user_info values(3,' corner の Wind chime ','27w4921',' Li Xiaofei ',9);
insert into address values(6,6,' Liu Qian ','13159775555',' Changchun, Jilin ');

Explicit Equivalent connection ：
Format ：

select ui.*,addr.*
from address addr 
join  user_info  ui   on  ui.id=addr.user_id;

Implicit Equivalent connection ：

Format ：

select ui.*,addr.* from user_info ui,address addr where ui.id=addr.user_id;

Although they have different formats , But the results are the same
result ：

External connection

External connection classification ：
1. Left outer link
2. Right outer link
3. Full outer join （ Because the total external connection is Oracle So here we just know ）

The left outer join ：

characteristic ： With left Keywords are references , The table on the left of the keyword is the main table , Display all its data , Even if there is no data from the table

Format ：

select ui.*,addr.*
from user_info ui
left join  address addr on ui.id =addr.user_id;

result ：

Right connection ：

characteristic ： With right Keywords are references , The table on the right of the keyword is the main table , Display all its data , Even if there is no data from the table

Format ：

select ui.*,addr.*
from address addr
right join user_info ui on ui.id=addr.user_id;

result ：

notes ： The left outer connection and the right outer connection can be transformed into each other .
example ：
The left outer join

select ui.*,addr.*
from address addr
left join user_info ui  on ui.id =addr.user_id;

Right connection

select ui.*,addr.*
from user_info ui
right join address addr  on ui.id=addr.user_id;

result ：