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Primary Key vs. Unique Key — What's the Difference?

By Tayyaba Rehman — Published on November 21, 2023
Primary Key uniquely identifies each record in a table and cannot be null; Unique Key also ensures uniqueness but can allow one null value.
Primary Key vs. Unique Key — What's the Difference?

Difference Between Primary Key and Unique Key


Key Differences

Both Primary Key and Unique Key play vital roles in relational database systems, ensuring data integrity. The Primary Key is a column or set of columns in a table that uniquely identifies every row of the table. It is the foremost key used to distinguish records and cannot have any null values. This key guarantees that every single entry in a database table is uniquely identifiable, ensuring data consistency.
On the other hand, a Unique Key is also a column or set of columns that uniquely identifies rows in a table. However, a significant difference is that while the Primary Key cannot have null values, the Unique Key can contain one null value for the column. This means a column defined as a Unique Key can have many records but only one that doesn't have a value (null).
In terms of constraints, a database table can have only one Primary Key. This key ensures that there are no duplicate records in the table. In contrast, a table can have more than one Unique Key. The function of the Unique Key is also to prevent duplicate entries, but it offers more flexibility with the null value exception.
Another distinction is their relationship with foreign keys. In database relationships, the Primary Key of one table can be referenced as the foreign key in another table. The Unique Key, while ensuring uniqueness, is not commonly used as a basis for foreign key relationships.
In summary, both Primary Key and Unique Key uphold the integrity of data in database systems. While they share the objective of maintaining unique data entries, their treatment of null values and constraints applied in database design are notably different.

Comparison Chart


Uniquely identifies each row
Ensures uniqueness but allows one null

Null Values

Does not allow nulls
Allows one null value

Number in a Table

One per table
Multiple allowed per table

Relationship Use

Commonly used for foreign keys
Not typically used for foreign keys


Ensures unique identification
Prevents duplicate entries with null exception

Compare with Definitions

Primary Key

A database element ensuring each entry is distinctive.
For customer records, the Primary Key could be their customer ID.

Unique Key

An attribute or set of attributes ensuring distinct records with an exception.
The optional landline number can be the Unique Key in an employee database.

Primary Key

A specific column that uniquely identifies each row.
The employee ID acts as the Primary Key in the employee database.

Unique Key

A column ensuring data uniqueness with a null exception.
A driver's license number can be a Unique Key in a system, with allowances for null.

Primary Key

The foremost constraint to ensure data uniqueness.
In a student database, the roll number serves as the Primary Key.

Unique Key

A database element enforcing non-redundant values with a null allowance.
A user's optional mobile number in a database can act as a Unique Key.

Primary Key

A non-redundant identifier for records in a table.
Every book has a Primary Key called ISBN in a library system.

Unique Key

An identifier similar to a primary key but can contain one null.
The passport number can be a Unique Key in a database, allowing one null entry.

Primary Key

A column or set of columns that cannot be null.
The Primary Key for a product might be its product code.

Unique Key

A constraint prohibiting duplicate data with a specific exception.
In a database of optional email IDs, the email column can be a Unique Key.

Common Curiosities

What's the main purpose of a Primary Key?

A Primary Key's purpose is to uniquely identify each record in a database table.

How do Primary Key and Unique Key differ in terms of null values?

Primary Key cannot have null values, while Unique Key can have one.

How many Primary Keys can a table have?

A table can have only one Primary Key.

Why is it crucial to have a Primary Key in a table?

A Primary Key ensures data integrity by uniquely identifying each record.

Can a Unique Key contain null values?

Yes, a Unique Key can contain one null value.

Can a table have multiple Unique Keys?

Yes, a table can have more than one Unique Key.

Can a Primary Key be a combination of columns?

Yes, a Primary Key can be a combination of multiple columns, known as a composite key.

Can a Unique Key become a Primary Key?

Yes, if it meets the Primary Key criteria, a Unique Key can be promoted to a Primary Key.

Does a Unique Key prevent duplicate entries?

Yes, a Unique Key prevents duplicate entries, but it allows one null value.

Are Primary Key values changeable?

Ideally, Primary Key values should remain constant to maintain data integrity.

What is the significance of allowing a null in Unique Key but not in Primary Key?

It provides flexibility in databases where a unique constraint is needed, but a value might be unknown.

Why can there be only one Primary Key in a table but multiple Unique Keys?

The Primary Key is the main identifier for records; multiple such primary identifiers would be redundant, whereas Unique Keys serve additional uniqueness constraints with more flexibility.

Which key, Primary Key or Unique Key, is commonly used for foreign key relationships?

The Primary Key is commonly used for foreign key relationships.

What happens if you try to insert a duplicate value in a Unique Key column?

An error will occur, preventing the insertion of duplicate values.

What happens to related data if a Primary Key value is deleted?

Deleting a Primary Key value can potentially delete related data in other tables if cascading delete is set.

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Author Spotlight

Written by
Tayyaba Rehman
Tayyaba Rehman is a distinguished writer, currently serving as a primary contributor to As a researcher in semantics and etymology, Tayyaba's passion for the complexity of languages and their distinctions has found a perfect home on the platform. Tayyaba delves into the intricacies of language, distinguishing between commonly confused words and phrases, thereby providing clarity for readers worldwide.

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