📜  在 SQL 中连接三个或更多表

📅  最后修改于: 2022-05-13 01:54:48.507000             🧑  作者: Mango

在 SQL 中连接三个或更多表

有时可能会出现需要从三个或更多表中获取数据的情况。本文介绍了实现它的两种方法。

例子:
创建三个表:

  1. 学生
  2. 分数
  3. 细节

注:图片不清晰可点击放大查看。

表一:学生

create table student(s_id int primary key, 
                      s_name varchar(20));

insert into student values(1, 'Jack');
insert into student values(2, 'Rithvik');
insert into student values(3, 'Jaspreet');
insert into student values(4, 'Praveen');
insert into student values(5, 'Bisa');
insert into student values(6, 'Suraj');

表 2:标记

create table marks(school_id int primary key, s_id int, 
                       score int, status varchar(20));

insert into marks values(1004, 1, 23, 'fail');
insert into marks values(1008, 6, 95, 'pass');
insert into marks values(1012, 2, 97, 'pass');
insert into marks values(1016, 7, 67, 'pass'); 
insert into marks values(1020, 3, 100, 'pass');
insert into marks values(1025, 8, 73, 'pass');
insert into marks values(1030, 4, 88, 'pass');
insert into marks values(1035, 9,  13, 'fail');
insert into marks values(1040, 5,  16, 'fail');
insert into marks values(1050, 10, 53, 'pass');

表 3:详细信息

create table details(address_city varchar(20), email_ID varchar(20), 
                      school_id int, accomplishments varchar(50));

insert into details values('Banglore',  'jsingh@geeks.com', 
                                1020, 'ACM ICPC selected');
insert into details values('Hyderabad', 'praveen@geeks.com', 
                                1030, 'Geek of the month');
insert into details values('Delhi',     'rithvik@geeks.com', 
                                    1012, 'IOI finalist');
insert into details values('Chennai',   'om@geeks.com', 
                                 1111, 'Geek of the year');
insert into details values('Banglore', ' suraj@geeks.com',
                                 1008, 'IMO finalist');
insert into details values('Mumbai',    'sasukeh@geeks.com',
                                  2211, 'Made a robot');
insert into details values('Ahmedabad', 'itachi@geeks.com',
                               1172, 'Code Jam finalist');
insert into details values('Jaipur',    'kumar@geeks.com',
                                   1972, 'KVPY finalist');

连接三个或更多表的两种方法:
1、在sql中使用joins来加入表:
应用相同的逻辑来连接 2 个表,即连接n 个表的最小连接语句数为(n-1)
询问:

select s_name, score, status, address_city, email_id,
accomplishments from student s inner join marks m on
s.s_id = m.s_id inner join details d on 
d.school_id = m.school_id;

输出:

2.使用父子关系:
这是一个相当有趣的方法。创建列X作为一个表中的主键和另一个表中的外键(即创建父子关系)。
让我们看看创建的表:
s_id是学生表中的主键,是标记表中的外键(学生(父母)-分数(孩子))
school_id是标记表中的主键和详细信息表中的外键(标记(父)-详细信息(子))

询问:

select s_name, score, status, address_city, 
email_id, accomplishments from student s, 
marks m, details d where s.s_id = m.s_id and 
m.school_id = d.school_id;

输出: