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Primary Key in DBMS vs. Unique Key in DBMS: Know the Difference

Shumaila Saeed
By Shumaila Saeed || Published on February 22, 2024
A Primary Key in DBMS uniquely identifies each record in a table and cannot be null, while a Unique Key also ensures record uniqueness but can allow one null value.
Primary Key in DBMS vs. Unique Key in DBMS

Key Differences

The Primary Key in a DBMS is a unique identifier for each record in a table, ensuring that no two records have the same value. The Unique Key also ensures uniqueness but unlike the primary key, it can be assigned to more than one column.
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A Primary Key cannot have null values, making every record uniquely identifiable. A Unique Key, however, can contain a single null value in a column, allowing for some flexibility.
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Each table in a DBMS can have only one Primary Key. In contrast, a table can have multiple Unique Keys, providing various ways to uniquely identify records.
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The Primary Key automatically creates a unique index for fast retrieval, while the Unique Key also creates an index, but its main purpose is to prevent duplicate values.
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The Primary Key is crucial for relationships between tables, as it defines foreign keys in other tables. The Unique Key, while it helps maintain data integrity, is not typically used in table relationships.
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Comparison Chart

Nullability

Cannot have null values
Can have one null value
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Number in a Table

Only one per table
Multiple allowed per table
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Purpose

Uniquely identifies each record
Prevents duplicate values
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Role in Relationships

Used to define foreign keys
Not typically used in relationships
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Index Creation

Automatically creates a unique index
Creates an index, not necessarily unique
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Primary Key in DBMS and Unique Key in DBMS Definitions

Primary Key in DBMS

Primary Key forms the basis for foreign keys in relational DBMS.
The account number in a banking system is a Primary Key, linked as a foreign key in transactions.
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Unique Key in DBMS

Unique Key provides additional means to maintain data integrity.
'PassportNumber' serves as a Unique Key in an international traveler database.
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Primary Key in DBMS

Primary Key in DBMS cannot contain null values.
In a products table, each product's unique identifier is a non-null Primary Key.
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Unique Key in DBMS

Unique Key ensures data uniqueness in a specific column.
The 'SerialNumber' of products in a warehouse database is a Unique Key.
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Primary Key in DBMS

A Primary Key uniquely identifies each row in a DBMS table.
The 'EmployeeID' column serves as the Primary Key in the employee database.
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Unique Key in DBMS

A table can have multiple Unique Keys in DBMS.
In a car database, both 'VIN' and 'LicensePlate' can be Unique Keys.
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Primary Key in DBMS

A Primary Key is a unique constraint in a DBMS table.
The order number in an order table acts as a Primary Key to uniquely identify orders.
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Unique Key in DBMS

Unique Key in DBMS ensures column values are distinct.
The 'Email' column is a Unique Key in a user database, preventing duplicate emails.
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Primary Key in DBMS

Primary Key ensures no two records in a table are identical.
In a student database, the 'StudentID' as a Primary Key differentiates every student.
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Unique Key in DBMS

Unique Key can have a null value, unlike a Primary Key.
A Unique Key 'AlternateEmail' can be null in a contact information table.
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Repeatedly Asked Queries

What is a Unique Key in DBMS?

It ensures that all values in a column are distinct.
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Can a Primary Key have null values?

No, a Primary Key cannot contain null values.
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What is a Primary Key in DBMS?

It uniquely identifies each record in a database table.
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How many Unique Keys can a table have?

A table can have multiple Unique Keys.
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How many Primary Keys can a table have?

Only one Primary Key per table.
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Is Primary Key necessary in a DBMS table?

Yes, for uniquely identifying records and relationships.
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What happens if a Primary Key is duplicated?

The DBMS will not allow duplication in a Primary Key.
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What if a Unique Key value is duplicated?

The DBMS will prevent duplication except for nulls.
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Can a Primary Key be a foreign key in another table?

Yes, it's commonly used as a foreign key.
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Can a Unique Key have null values?

Yes, it can have one null value per column.
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Does a Primary Key create an index?

Yes, it automatically creates a unique index.
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What are the benefits of using a Unique Key?

It prevents duplicate values and maintains data uniqueness.
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Is Unique Key necessary in a table?

It's not necessary but helps maintain data integrity.
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What are the benefits of using a Primary Key?

It ensures record uniqueness and relational integrity.
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Can a Primary Key be composite?

Yes, it can consist of multiple columns.
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Can a Unique Key be composite?

Yes, it can also consist of multiple columns.
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Can Primary and Unique Keys coexist in a table?

Yes, they can coexist and serve different purposes.
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Are Primary and Unique Keys always columns?

They are constraints applied to columns or sets of columns.
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Does a Unique Key create an index?

Yes, it creates an index but not necessarily unique.
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Can a Unique Key be used as a foreign key?

It's less common but possible.
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Shumaila Saeed
Written by
Shumaila Saeed
Shumaila Saeed, an expert content creator with 6 years of experience, specializes in distilling complex topics into easily digestible comparisons, shining a light on the nuances that both inform and educate readers with clarity and accuracy.

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