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Joining three or more tables in SQL
  • Difficulty Level : Medium
  • Last Updated : 03 May, 2020

There may occur some situations sometimes where data needs to be fetched from three or more tables. This article deals with two approaches to achieve it.

Example:
Creating three tables:

  1. student
  2. marks
  3. details

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Table 1: student

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');



Table 2: marks

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');

Table 3: details

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');

Two approaches to join three or more tables:
1. Using joins in sql to join the table:
The same logic is applied which is done to join 2 tables i.e. minimum number of join statements to join n tables are (n-1).
Query:

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;

Output:

2. Using parent-child relationship:
This is rather an interesting approach. Create column X as primary key in one table and as foreign key in another table (i.e creating a parent-child relationship).
Let’s look in the tables created:
s_id is the primary key in student table and is foreign key in marks table. (student (parent) – marks(child)).
school_id is the primary key in marks table and foreign key in details table. (marks(parent) – details(child)).

Query:

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;

Output:

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