What is a foreign key?

What is a foreign key?

A foreign key means that values in one table must also appear in another table.

The referenced table is called the parent table while the table with the foreign key is called the child table. The foreign key in the child table will generally reference a primary key in the parent table.

A foreign key can be defined in either a CREATE TABLE statement or an ALTER TABLE statement.

Using a CREATE TABLE statement

The syntax for creating a foreign key using a CREATE TABLE statement is:

CREATE TABLE table_name

(

column1 datatype null/not null,

column2 datatype null/not null,

...

CONSTRAINT fk_column

FOREIGN KEY (column1, column2, ... column_n)

REFERENCES parent_table (column1, column2, ... column_n)

);

For Example

CREATE TABLE supplier

( supplier_id numeric(10) not null,

supplier_name varchar2(50) not null,

contact_name varchar2(50),

CONSTRAINT supplier_pk PRIMARY KEY (supplier_id)

);

CREATE TABLE products

( product_id numeric(10) not null,

supplier_id numeric(10) not null,

CONSTRAINT fk_supplier

FOREIGN KEY (supplier_id)

REFERENCES supplier(supplier_id)

);

In this example, we've created a primary key on the supplier table called supplier_pk. It consists of only one field - the supplier_id field. Then we've created a foreign key called fk_supplier on the products table that references the supplier table based on the supplier_id field.

We could also create a foreign key with more than one field as in the example below:

CREATE TABLE supplier<

( supplier_id numeric(10) not null,

supplier_name varchar2(50) not null,

contact_name varchar2(50),

CONSTRAINT supplier_pk PRIMARY KEY (supplier_id, supplier_name)

);

CREATE TABLE products

( product_id numeric(10) not null,

supplier_id numeric(10) not null,

supplier_name varchar2(50) not null,

CONSTRAINT fk_supplier_comp

FOREIGN KEY (supplier_id, supplier_name)

REFERENCES supplier(supplier_id, supplier_name)

);

In this example, our foreign key called fk_foreign_comp references the supplier table based on two fields - the supplier_id and supplier_name fields.

Using an ALTER TABLE statement

The syntax for creating a foreign key in an ALTER TABLE statement is:

ALTER TABLE table_name

add CONSTRAINT constraint_name

FOREIGN KEY (column1, column2, ... column_n)

REFERENCES parent_table (column1, column2, ... column_n);

For Example

ALTER TABLE products

add CONSTRAINT fk_supplier

FOREIGN KEY (supplier_id)

REFERENCES supplier(supplier_id);

In this example, we've created a foreign key called fk_supplier that references the supplier table based on the supplier_id field.

We could also create a foreign key with more than one field as in the example below:

ALTER TABLE products

add CONSTRAINT fk_supplier

FOREIGN KEY (supplier_id, supplier_name)

REFERENCES supplier(supplier_id, supplier_name);

Others: Department and Students Table

create table department
(dept_id numeric(10) not null,
dept_name varchar2(30) not null,
constraint department_pk primary key(dept_id)
); 

 
create table students
(std_id numeric(10) not null,
std_name varchar2(30) not null,
dept_id numeric(10) not null,
constraint student_pk primary key(std_id),
constraint fk_department
foreign key (dept_id)
references department(dept_id)
);