Primary Key vs Foreign Key in DBMS (With Examples)

Introduction

In relational databases, Primary Keys and Foreign Keys are essential for organizing data and creating relationships between tables. Many students confuse these two concepts, but the difference is simple once you understand their roles.

This guide explains Primary Key vs Foreign Key clearly with examples.

 

What is a Primary Key?

A Primary Key is a column (or set of columns) that uniquely identifies each record in a table.

 It ensures that no two rows have the same identity.

Key Features of Primary Key

• Must contain unique values
• Cannot contain NULL values
• Only one primary key per table
• Used to identify each record

Example

Students table

StudentID	Name	Age
101	Ali	21
102	Sara	22

 Here, StudentID is the Primary Key because it uniquely identifies each student.

 

What is a Foreign Key?

A Foreign Key is a column in one table that refers to the Primary Key in another table.

 It is used to create a relationship between tables.

Key Features of Foreign Key

• Can contain duplicate values
• Can contain NULL values (in most cases)
• Multiple foreign keys allowed in a table
• Maintains referential integrity

 

Example of Foreign Key

Students table (Parent table)

StudentID (PK)	Name
101	Ali
102	Sara

Orders table (Child table)

OrderID	StudentID (FK)	Course
1	101	DBMS
2	101	SQL
3	102	Networking

 Here:

• StudentID in Students → Primary Key
• StudentID in Orders → Foreign Key

This links orders to the correct student.

 

Primary Key vs Foreign Key (Comparison Table)

Feature	Primary Key	Foreign Key
Purpose	Uniquely identifies records	Links two tables
Uniqueness	Must be unique	Can have duplicates
NULL values	Not allowed	Allowed (usually)
Number per table	Only one	Multiple allowed
Location	Same table it identifies	References another table
Role	Parent identifier	Relationship creator

 

Simple Real-Life Analogy

Think of:

• Primary Key → Student NIC number (unique identity)
• Foreign Key → Student ID written in exam records

The foreign key points back to the student’s unique identity.

 

SQL Example

Create Students Table

CREATE TABLE Students (
  StudentID INT PRIMARY KEY,
  Name VARCHAR(50)
);

Create Orders Table with Foreign Key

CREATE TABLE Orders (
  OrderID INT PRIMARY KEY,
  StudentID INT,
  Course VARCHAR(50),
  FOREIGN KEY (StudentID) REFERENCES Students(StudentID)
);

 

Why These Keys Are Important

Primary Key helps to:

• Uniquely identify data
• Prevent duplicate records
• Improve indexing

Foreign Key helps to:

• Maintain relationships
• Ensure data consistency
• Prevent invalid data entry

 

Conclusion

A Primary Key uniquely identifies each record in a table, while a Foreign Key creates relationships between tables by referencing the Primary Key of another table. Both are essential for maintaining organized, accurate, and relational databases.