Primary Key vs. Candidate Key — What's the Difference?
By Tayyaba Rehman & Fiza Rafique — Published on February 3, 2024
A Primary Key uniquely identifies each record in a database table, whereas a Candidate Key is any key that can serve as a Primary Key.
Difference Between Primary Key and Candidate Key
Table of Contents
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Key Differences
A Primary Key in database management is a unique identifier for each record in a table, ensuring no two records are identical. A Candidate Key is also capable of uniquely identifying each record but isn't necessarily chosen as the Primary Key.
Every table must have one and only one Primary Key, selected from one of the Candidate Keys. Candidate Keys are potential Primary Keys, and a table can have multiple Candidate Keys.
The Primary Key cannot have null values and must contain unique values. In contrast, Candidate Keys also must uniquely identify records but can remain unused as the primary identifier.
The selection of a Primary Key is crucial for database integrity and is used in defining relationships between tables. Candidate Keys are identified during database design but may or may not be used as a Primary Key.
A Primary Key can be a single column or a combination of columns (composite key), as long as it maintains uniqueness. Candidate Keys also follow this rule but are not limited to being selected as the primary means of record identification.
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Comparison Chart
Definition
Unique identifier for each record in a table
A key that can uniquely identify each record, potential to be a Primary Key
Uniqueness
Must be unique
Must be unique
Null Values
Cannot contain nulls
Cannot contain nulls
Quantity in Table
One per table
One or more per table
Role in Database
Used for record identification and relationships
Identified during design, may or may not be used as Primary Key
Compare with Definitions
Primary Key
The Primary Key is essential for maintaining database integrity.
In a student database, the student registration number is used as the Primary Key.
Candidate Key
Candidate Keys must be unique and not contain null values.
The 'License Plate' and 'Vehicle Identification Number (VIN)' are Candidate Keys in a vehicle registration database.
Primary Key
The Primary Key is used to establish relationships between tables.
The 'Customer ID' as a Primary Key links customer orders to customer details.
Candidate Key
A Candidate Key can uniquely identify database records and qualify as a Primary Key.
Both 'Email' and 'Phone Number' could be Candidate Keys in a contact database.
Primary Key
A Primary Key is a unique identifier that cannot have null values.
A product's serial number acts as a Primary Key in a product inventory database.
Candidate Key
Candidate Keys are potential Primary Keys in a database.
In a book database, both 'ISBN' and 'Title-Author' combination could be Candidate Keys.
Primary Key
A Primary Key can be a single field or a combination of fields.
A car's make and model together can serve as a Primary Key in an automobile database.
Candidate Key
A table can have multiple Candidate Keys.
An employee's 'Social Security Number' and 'Employee ID' can both be Candidate Keys.
Primary Key
A Primary Key uniquely identifies each record in a database table.
The 'Employee ID' field serves as the Primary Key in an employee database.
Candidate Key
Candidate Keys are identified during database design.
In an airline reservation system, 'Passport Number' and 'Ticket Number' might be identified as Candidate Keys.
Common Curiosities
Can a table have more than one Primary Key?
No, a table can only have one Primary Key.
What is a Candidate Key?
A Candidate Key is a key that can uniquely identify each record in a table and can potentially be a Primary Key.
What is a Primary Key?
A Primary Key is a unique identifier for each record in a database table.
Are Candidate Keys always used as Primary Keys?
No, not all Candidate Keys are used as Primary Keys.
Is a Primary Key always a single column?
No, a Primary Key can be a single column or a combination of columns (composite key).
How many Candidate Keys can a table have?
A table can have multiple Candidate Keys.
Can a Primary Key be changed?
Changing a Primary Key is possible but can be complex and impact database integrity.
Can a Candidate Key contain null values?
No, like a Primary Key, a Candidate Key cannot contain null values.
Can a Candidate Key become a Primary Key?
Yes, a Candidate Key can be chosen as the Primary Key.
What happens if a Primary Key is duplicated?
Duplicate Primary Keys are not allowed as they violate the uniqueness requirement.
Why is a Primary Key important in a database?
A Primary Key is crucial for uniquely identifying records and establishing relationships between tables.
Do all tables need a Primary Key?
Ideally, yes, to maintain data integrity and enable relationships between tables.
Can foreign keys be Candidate Keys?
Yes, foreign keys can be Candidate Keys if they uniquely identify records in their own table.
How is a Primary Key chosen from Candidate Keys?
The Primary Key is chosen based on its ability to uniquely identify records and database design requirements.
Can a Primary Key be a foreign key in another table?
Yes, a Primary Key can also serve as a foreign key in a different table, linking the two tables.
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Written by
Tayyaba RehmanTayyaba Rehman is a distinguished writer, currently serving as a primary contributor to askdifference.com. 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.
Co-written by
Fiza RafiqueFiza Rafique is a skilled content writer at AskDifference.com, where she meticulously refines and enhances written pieces. Drawing from her vast editorial expertise, Fiza ensures clarity, accuracy, and precision in every article. Passionate about language, she continually seeks to elevate the quality of content for readers worldwide.
