What is the Difference Between Primary Key and Unique Key?

Primary key and unique key are database constraints used to enforce uniqueness in SQL tables.

In simple words:

• Primary key uniquely identifies each row and cannot contain NULL values
• Unique key also enforces uniqueness but can allow NULL values depending on the database

Main Difference Between Primary Key and Unique Key

What is a Primary Key?

A primary key is a column or combination of columns used to uniquely identify each row in a table.

Main Features of Primary Key

• Must contain unique values
• Cannot contain NULL values
• Only one primary key per table

Example

CREATE TABLE employees (

    employee_id INT PRIMARY KEY,

    employee_name VARCHAR(100)

);

Sample Data

Invalid Cases

Duplicate Value

employee_id = 1
employee_id = 1

NULL Value

employee_id = NULL

Database Rejects These

• Because primary key must be unique and NOT NULL

What is a Unique Key?

A unique key is a constraint used to ensure that values in a column remain unique across rows.

Main Features of Unique Key

• Ensures uniqueness
• Can allow NULL values in many databases
• Multiple unique keys allowed per table

Example

CREATE TABLE users (

    user_id INT PRIMARY KEY,

    email VARCHAR(100) UNIQUE

);

Sample Data

Invalid Case

naresh@gmail.com
naresh@gmail.com

Problem

• Duplicate email not allowed

Primary Key Internal Architecture

Table Row
    |
    v
Primary Key Value
    |
    v
Uniquely Identifies Record

Unique Key Internal Architecture

Column Values
     |
     v
Uniqueness Validation
     |
     v
Duplicate Prevention

Primary Key Query Flow

Insert Row
    |
    v
Check Uniqueness
    |
    v
Check NOT NULL
    |
    +---- Valid ----> Insert Success
    |
    +---- Invalid ----> Error

Unique Key Query Flow

Insert Row
    |
    v
Check Duplicate Values
    |
    +---- Unique ----> Insert Success
    |
    +---- Duplicate ----> Error

Primary Key vs Unique Key with NULL Example

Primary Key

employee_id = NULL

Result

• Error

Unique Key

email = NULL

Result

• Allowed in many databases

Composite Primary Key

Primary key can contain:

• Multiple columns

Example

PRIMARY KEY(student_id, course_id)

Purpose

• Uniquely identify combined rows

Composite Unique Key

Unique key can also contain:

• Multiple columns

Example

UNIQUE(first_name, phone_number)

Primary Key vs Unique Key Performance

Primary Key in Banking Systems

Banking systems use primary keys for:

• Account IDs
• Transaction IDs
• Customer IDs

Why?

• Every record must be uniquely identifiable

Unique Key in Banking Systems

Banking systems use unique keys for:

• Email addresses
• PAN numbers
• Passport numbers

Example

Each customer email must be unique

Primary Key in E-Commerce

E-commerce systems use primary keys for:

• Order IDs
• Product IDs
• User IDs

Unique Key in E-Commerce

E-commerce systems use unique keys for:

• Product SKU codes
• User emails
• Coupon codes

Primary Key in Learning Platforms

Learning systems use primary keys for:

• Student IDs
• Course IDs
• Assessment IDs

Unique Key in Learning Platforms

Learning systems use unique keys for:

• Student email IDs
• Certificate numbers
• Username validation

Primary Key in Microservices

Microservices architectures use primary keys for:

• Entity identification
• Distributed database references
• API entity mapping

Unique Key in Microservices

Microservices use unique keys for:

• User authentication identifiers
• External system mapping
• Business uniqueness rules

Advantages of Primary Key

• Ensures entity uniqueness
• Improves indexing performance
• Supports referential integrity
• Required for normalization

Advantages of Unique Key

• Prevents duplicate business data
• Allows multiple unique constraints
• Supports business rules

Disadvantages of Primary Key

• Only one allowed per table
• Changing primary key may affect relationships

Disadvantages of Unique Key

• NULL handling differs across databases
• Additional indexes increase storage usage

Best Practices

• Use stable primary keys
• Avoid changing primary key values
• Use unique keys for business constraints
• Index important unique columns
• Maintain referential integrity

Common Interview Mistake

Many developers think:

• Primary key and unique key are exactly the same

Reality

Primary key:

• Is the main identifier and cannot contain NULL values

Unique key:

• Mainly enforces uniqueness and may allow NULL values

Related Learning Topics

• What is a Primary Key?
• What is a Foreign Key?
• What is Referential Integrity?
• Database Normalization
• Indexing and Performance

Professional Interview Answer

A primary key is a database constraint used to uniquely identify each row in a table and does not allow NULL values. Each table can have only one primary key, although it may consist of multiple columns as a composite primary key. A unique key is a constraint that ensures uniqueness of values in one or more columns but can allow NULL values depending on the database implementation. Unlike primary keys, multiple unique keys can exist in the same table. Primary keys are commonly used for entity identification and referential integrity, while unique keys are mainly used to enforce business rules such as unique email addresses, usernames, product codes, or certificate numbers. Enterprise systems such as banking platforms, e-commerce applications, ERP systems, learning management systems, and microservices architectures extensively use both primary and unique keys for consistency, indexing, and relational integrity.

Why Interviewers Like This Answer

• Clearly differentiates both constraints
• Mentions NULL handling
• Includes indexing and referential integrity concepts
• Provides enterprise-level examples
• Demonstrates strong relational database knowledge

Frequently Asked Questions

What is a primary key?

A primary key uniquely identifies each row in a table and cannot contain NULL values.

What is a unique key?

A unique key ensures that column values remain unique across rows.

Can a table have multiple primary keys?

No, a table can have only one primary key.

Can a table have multiple unique keys?

Yes, multiple unique keys are allowed.

Does unique key allow NULL values?

Yes, many databases allow NULL values in unique keys.