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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 foreign key
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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