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Primary key vs. Unique key: What's the Difference?

Edited by Aimie Carlson || By Janet White || Published on March 5, 2024
A primary key uniquely identifies each record in a database table and cannot be null, while a unique key also ensures records are unique but can contain null values.

Key Differences

A primary key is a unique identifier for each record in a database table, ensuring that no two rows have the same primary key value. It is a fundamental aspect of relational database design, used to establish and enforce entity integrity. A unique key also enforces uniqueness for the column or columns on which it is defined, ensuring that no two rows of a table have the same values in those columns. Unlike a primary key, a unique key can accept null values, and more than one unique key can be defined on a table.
Both primary keys and unique keys are essential for relational database management, playing crucial roles in maintaining data integrity and facilitating efficient data retrieval. The primary key serves as the main means of identifying a row within a table, often used in foreign key relationships to link tables together. Unique keys, while also ensuring data uniqueness, provide additional flexibility in database design, allowing for multiple unique constraints within a single table.
The selection of a primary key is critical to database design, often influencing the structure and access patterns of the database. It is chosen to uniquely represent each row in a table, with considerations for performance and data integrity. In contrast, unique keys are used to enforce business rules and constraints that require uniqueness beyond the primary key, such as email addresses or social security numbers.
Understanding the distinction between primary keys and unique keys is fundamental to effective database schema design, impacting how data relationships are defined and how integrity constraints are enforced. Both keys play pivotal roles in ensuring the accuracy and reliability of the data stored within a database, albeit with different constraints and implications for data modeling.

Comparison Chart


Cannot contain null values.
Can contain null values (one null per unique key allowed).


Must be unique across all records.
Must be unique across all records but can have nulls.

Quantity per Table

Only one primary key allowed per table.
Multiple unique keys can be defined per table.


Uniquely identifies each record in a table.
Ensures data uniqueness, not necessarily used to identify records.

Foreign Key Relationship

Often used as a reference in foreign key relationships.
Rarely used in foreign key relationships compared to primary keys.

Primary key and Unique key Definitions

Primary key

Cannot be null and ensures entity integrity.
Each product's serial number is a primary key ensuring no two products have the same identifier.

Unique key

Ensures all values in a column are unique, allowing nulls.
The email address is a unique key in the users table, ensuring no two users share the same email.

Primary key

A unique identifier for each record in a database table.
The customer ID serves as the primary key in the customer table.

Unique key

Additional constraint to enforce uniqueness besides the primary key.
Employee IDs are unique keys in addition to the departmental primary keys.

Primary key

Used in relational databases to link tables together.
The student ID is the primary key, used to link students with their course enrollments.

Unique key

Can be applied to multiple columns in a table.
A combination of first name and last name could be a unique key in a contacts table.

Primary key

Unique across the entire table and defines record uniqueness.
The order ID is the primary key, uniquely identifying each order.

Unique key

Flexible in allowing multiple unique constraints within a single table.
Both passport number and national ID can be unique keys in a person's database record.

Primary key

The central element of a table's structure and relationships.
In the database schema, the account number is defined as the primary key for the accounts table.

Unique key

Prevents duplicate values in a specific column or set of columns.
Social security numbers are stored in a column defined by a unique key.


Can a primary key be a composite key?

Yes, a primary key can consist of two or more columns to ensure uniqueness.

How many unique keys can a table have?

A table can have multiple unique keys.

What is a unique key?

A constraint that ensures all values in a column or group of columns are unique, with allowance for null values.

Can a table have more than one primary key?

No, a table can have only one primary key.

Why is a primary key important in a database?

It uniquely identifies each record, ensuring data integrity and enabling efficient data retrieval.

Is it necessary to always define a primary key in a table?

Yes, for relational databases, defining a primary key is crucial for data integrity and relationships.

How do null values affect unique keys?

Unique keys allow for a single null value for each unique constraint, except when defined otherwise.

How do primary keys and unique keys affect database performance?

Properly indexed, they can significantly improve query performance by enabling faster data retrieval.

What is a primary key?

A unique identifier for each record in a database table that cannot be null.

How does a primary key enhance data integrity?

By preventing duplicate and null values, ensuring each record is unique and identifiable.

What role do unique keys play in database design?

They enforce business rules by ensuring uniqueness for certain columns, supplementing the primary key constraint.

What happens if a primary key constraint is violated?

The database system will not allow the insertion or update of records that violate the primary key constraint.

What is the impact of deleting a primary key value?

Deleting a record with a primary key value removes that record from the table, affecting any related foreign key references.

Can a unique key be used as a foreign key?

Yes, though it's less common than using a primary key for this purpose.

Can a column be part of a primary key and a unique key at the same time?

Yes, a column can be part of both, but it's more common for it to be part of one due to the inherent uniqueness of a primary key.

Why might a database designer use both primary keys and unique keys in a table?

To uniquely identify records with a primary key while enforcing additional uniqueness constraints on other columns with unique keys, ensuring comprehensive data integrity.

What makes a unique key different from a primary key?

Unlike a primary key, a unique key can contain null values and a table can have multiple unique keys.

What are the benefits of having multiple unique keys?

They allow for additional unique constraints on different columns, useful for enforcing various business rules.

How do you choose a primary key for a table?

Choose a column (or set of columns) that uniquely identifies each row and is unlikely to change over time.

What are the implications of changing a primary key?

Changing a primary key can have significant implications, especially if it's used in foreign key relationships, requiring updates to all related tables.
About Author
Written by
Janet White
Janet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.
Edited by
Aimie Carlson
Aimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.

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