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How to create foreign key

PHP MySQL Foreign Key

What is foreign key

A column is called foreign key when it refers to the primary key column of another table.
More than one(multiple) tables can be linked to each other by one or more common fields, called foreign keys.
foreign keys make it possible to create one-to-one or one-to-many relationships between different tables, and combine data from multiple tables.

Before defining foreign key we have to create Primary key table. in this example we are creating a table "users" with 4 fields. first field "u_id" is primary key.

How to define Primary key

create table users
(
u_id int primary key auto_increment,
name char(50) not null,    
email varchar(50) unique key,
course char(50)
);

Output

How to define foreign key

create table fees
(
fee_id int primary key auto_increment,
paid_fee int not null,
due_fee int not null,
paid_date date not null,
u_id int not null,
foreign key(u_id) references users(u_id)    
);

Output

In this example we have create a table "fees" with 5 fields. in this table "fee_id" is primary key and u_id is foreign key which refers primary key of first table(users).

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