Difference Between Super Key and Candidate Key

Difference Between Super Key and Candidate Key

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Esha
Esha Gupta
Associate Senior Executive
Updated on Apr 2, 2024 11:53 IST

Do you know the difference between a candidate key and a super key? While both can uniquely identify rows in a table, a candidate key is the most refined version of a super key, containing no extra attributes beyond those necessary for unique identification. In contrast, a super key may include additional, non-essential attributes. Let's understand more!

A super key and a candidate key are concepts related to ensuring data uniqueness within a table. However, they serve different roles and have distinct characteristics. In this blog, we will understand the differences between them in detail!

Table of Content

Difference Between Super Key and Candidate Key

Below is a table differentiating between Super Key and Candidate Key.

Aspect

Super Key

Candidate Key

Definition

A super key is a set of one or more attributes that, taken collectively, can uniquely identify a tuple (row) in a relation (table).

A candidate key is a minimal super key, meaning it is a super key with no redundant attributes; it cannot have fewer attributes without losing the ability to uniquely identify a tuple.

Uniqueness

Every super key can uniquely identify each tuple in a relation.

Every candidate key can uniquely identify each tuple in a relation.

Minimality

A super key may or may not be minimal; it can contain extra attributes not necessary for unique identification.

A candidate key is minimal; it has the smallest possible number of attributes necessary for unique identification.

Redundancy

Super keys can contain redundant attributes.

Candidate keys contain no redundant attributes.

Number of Keys

There can be multiple super keys in a relation, each comprising a different combination of attributes.

There can be multiple candidate keys in a relation, but each is minimal.

Primary Key Selection

A super key is not necessarily a good candidate for selection as a primary key due to possible redundancy.

A candidate key is a potential choice for a primary key since it is minimal and can uniquely identify tuples.

 

What is a Super Key?

A Super Key is a set of one or more attributes (columns) that can uniquely identify a record (row) in a table (relation). The uniqueness criterion means that no two rows can have the same values for those attributes that make up the super key.

Characteristics of a Super Key

  • Every super key must uniquely identify each row in a table. This is the fundamental property that defines a super key.
  •  A super key can consist of any combination of columns that, in conjunction, guarantee the uniqueness of the row. Because of this, a table can have multiple super keys.
  • Every candidate key is a super key, but not every super key is a candidate key. A candidate key is a minimal super key, meaning it has no redundant attributes that are not needed for uniqueness.
  • While any candidate key can serve as a primary key, super keys as a whole are not suitable for this purpose unless they are minimal (which would then make them candidate keys).

What is a Candidate Key?

A Candidate Key is a concept in relational database theory that refers to a set of attributes within a table that uniquely identifies each row of the table. 

Important Properties of a Candidate Key

  • Every value of a candidate key must be unique across all rows of the table, ensuring that no two rows have the same value for the candidate key attributes.
  • A candidate key is minimal, meaning that no subset of the candidate key can be a candidate key. In other words, every attribute in the candidate key is necessary to maintain the key's uniqueness property.
  • Any candidate key can be chosen as the primary key for the table. The primary key is a special candidate key that the database designer selects to identify rows uniquely.

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Understanding the difference between these two types of keys is essential for effective database schema design, impacting data integrity, query performance, and overall database management. Keep learning, keep exploring!

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FAQs

What is a Super Key?

A Super Key is an attribute or a set of attributes within a database table that can uniquely identify each record in that table. Super Keys can consist of a single column or multiple columns combined. They include additional attributes that are not necessarily needed to ensure uniqueness. Essentially, any key that can uniquely identify a record can be considered a Super Key.

What is a Candidate Key?

A Candidate Key is a minimal Super Key, meaning it is a Super Key without any unnecessary attributes. In other words, every attribute in a Candidate Key is necessary to ensure uniqueness of the dataset. A table can have one or more Candidate Keys, and each Candidate Key can uniquely identify each record in the table.

How do Super Key and Candidate Key differ?

The main difference between a Super Key and a Candidate Key is the minimality. While both can uniquely identify records in a table, a Super Key may contain extra attributes that are not required for uniqueness. In contrast, a Candidate Key is a minimal set of attributes needed to ensure uniquenes, it does not have any spare or unnecessary attributes. In essence, every Candidate Key is a Super Key, but not every Super Key is a Candidate Key.

Can a table have multiple Candidate Keys? How does it affect the database design?

Yes, a table can have multiple Candidate Keys. Each Candidate Key can uniquely identify records in the table without redundancy. When a table has multiple Candidate Keys, one of them can be selected as the Primary Key. The choice of the Primary Key from among the Candidate Keys can affect the database's design and query performance, as the Primary Key is often used in indexing and as a reference for foreign keys in other tables.

How does a Primary Key relate to Super Keys and Candidate Keys?

A Primary Key is a special type of Candidate Key that is officially chosen to uniquely identify records in a table. While there can be multiple Candidate Keys in a table, only one can be chosen as the Primary Key. By definition, a Primary Key is also a Candidate Key and therefore a Super Key. It must satisfy the conditions of being unique and minimal, ensuring no part of the key is redundant. The selection of a Primary Key is crucial for database integrity, design, and the efficient retrieval of data.

About the Author
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Esha Gupta
Associate Senior Executive

Hello, world! I'm Esha Gupta, your go-to Technical Content Developer focusing on Java, Data Structures and Algorithms, and Front End Development. Alongside these specialities, I have a zest for immersing myself in v... Read Full Bio