Super Key vs. Candidate Key — What's the Difference?
By Tayyaba Rehman & Fiza Rafique — Published on February 1, 2024
Super Key is any set of attributes that can uniquely identify a row in a database table. Candidate Key is a minimal Super Key, having no redundant attributes.
Difference Between Super Key and Candidate Key
Table of Contents
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Key Differences
A Super Key in a database is a set of one or more columns that can uniquely identify records in a table. However, a Super Key may contain additional attributes that are not necessary for unique identification. A Candidate Key is a type of Super Key but is minimal, meaning it has no extraneous attributes; every part of it is needed to ensure uniqueness.
A Super Key can have redundant attributes that do not contribute to its uniqueness. In contrast, a Candidate Key is a Super Key stripped of any redundancy. It cannot be reduced further without losing the ability to uniquely identify each record.
In a table with columns [StudentID, Name, Address], both [StudentID, Name] and [StudentID, Address] could be Super Keys. However, [StudentID] alone would be a Candidate Key as it is the minimal set needed for unique identification.
There can be multiple Super Keys in a table, including all variations that include at least one Candidate Key. However, there are fewer Candidate Keys because they are minimal Super Keys. Each table may have one or more Candidate Keys.
Super Keys are considered during the initial stages of database design to understand all possible unique identifiers. Candidate Keys are more crucial as they are considered for further normalization processes and to set primary keys.
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Comparison Chart
Definition
Any set of attributes that ensures uniqueness.
A minimal set ensuring uniqueness.
Redundancy
Can contain redundant attributes.
No redundant attributes.
Number in a Table
Can be many.
Fewer, as they are minimal Super Keys.
Example
[StudentID, Name] in a student table.
[StudentID] in the same table.
Role in Normalization
Identified in initial design stages.
Used for setting primary keys.
Compare with Definitions
Super Key
Combination of Columns
Combining name and date of birth can form a Super Key in a customer database.
Candidate Key
Non-Redundant
In a Candidate Key, every attribute is essential for uniqueness.
Super Key
Unique Identification
A Super Key ensures that each row in the table is distinct.
Candidate Key
Crucial for Normalization
Candidate Keys are vital for database normalization and design.
Super Key
Flexible in Composition
Super Keys can vary in composition, like either [EmployeeID] or [EmployeeID, Department].
Candidate Key
Potential Primary Key
Any Candidate Key, such as [EmployeeID], can be chosen as the primary key.
Super Key
Broad Identifier
A Super Key, like [StudentID, Name], can uniquely identify student records.
Candidate Key
Minimal Uniqueness
A Candidate Key, like [StudentID], is the simplest key that can uniquely identify a record.
Super Key
Potentially Redundant
A Super Key may include extra data, like adding an address to the StudentID.
Candidate Key
Limited in Number
Unlike Super Keys, there are fewer Candidate Keys in a table.
Common Curiosities
Can a table have multiple Super Keys?
Yes, a table can have many Super Keys.
What is a Super Key?
A set of attributes that can uniquely identify each row in a database table.
Can a Super Key have extra attributes?
Yes, it can include unnecessary attributes.
How is a Candidate Key different from a Super Key?
It is the simplest possible key for unique identification.
Is a primary key a Candidate Key?
Yes, a primary key is a chosen Candidate Key.
Are all Candidate Keys also Super Keys?
Yes, every Candidate Key is a type of Super Key.
Can a Super Key become a primary key?
Only if it's also a Candidate Key.
What is a Candidate Key?
A minimal Super Key without redundant attributes.
How many Candidate Keys can a table have?
A table can have one or more Candidate Keys.
Why are Super Keys important in databases?
They help identify all possible unique identifiers.
Are Candidate Keys used in normalization?
Yes, they're crucial for database normalization.
Can a Candidate Key have composite attributes?
Yes, if they're necessary for uniqueness.
Do Super Keys help in identifying relationships?
Yes, they can help in identifying relationships between tables.
Is every column in a Candidate Key needed?
Yes, all parts of a Candidate Key are necessary.
How are Candidate Keys selected?
Based on their ability to uniquely and minimally identify records.
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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.
