SQL Foriegn Key

A Foreign Key is a primary key in Another table, which is to be referred in current table.
This helps us in separating tables and maintain relationships between them.
Example: We can have a department table and maintain a foreign key in employee table to link him to the department.
Example: Consider the following Employee table:

Employee Table

Rambo

Robert

Mugambo

Satraj

Nagashekar

Rao

Tom

Harry

Dana

Laura

Rambo

Fox

Giselle

Chivvi

Consider the following Department table:

Department Table

Engineering

Management

Finance

Kitchen

The Employee table has a DEPARTMENTID column which links to DEPARTMENTID column in the DEPARTMENT table.
The DEPARTMENTID column in Employee table is a foreign key.
The DEPARTMENTID column in Department table is a primary key.
A foriegn key can be created at the time of creating a table.

CREATE TABLE EMPLOYEE(
		EMPID INT NOT NULL,
		EMPNAME VARCHAR(50) NOT NULL,
		EMPDESIGNATION VARCHAR(50),
		PRIMARY KEY(EMPID),
		FOREIGN KEY(DEPARTMENTID) REFERENCES DEPARTMENT(DEPARTMENTID);
		);

Alternatively, a foreign key can be created at a later time, using ALTER command.

ALTER TABLE EMPLOYEE
		ADD CONSTRAINT FKDEPARTMENT 
		FOREIGN KEY (DEPARTMENTID) REFERENCES DEPARTMENT(DEPARTMENTID);