Difference Between Primary Key and Candidate Key

By Anjnee Bhatnagar|Updated : September 20th, 2022

The difference between primary key and candidate key is that the primary key is a minimal super key. Here, we need to understand the meaning of these key types individually before discussing the difference between primary key and candidate key in detail. However, both primary and candidate key helps the user to identify a tuple uniquely in the relation. There is a subtle difference between primary and candidate keys, which will get clear as we proceed further.

In this article, we will learn about the primary key, candidate key, and the difference between the primary key and candidate key in detail. The attributes that are utilized to access tuples from a table are Primary Key and Candidate Key. Additionally, these keys can be utilized to establish a connection between two tables.

Difference Between Primary Key and Candidate Key PDF

Difference Between Primary Key and Candidate Key

We will discuss the primary and candidate keys individually, but let us check the difference between primary key and candidate key, which are depicted in the table below.

Primary Key VS Candidate Key

Primary Key

Candidate Key

It is used to uniquely identify a tuple in the relation.

It is the minimal set of attributes used to identify a tuple uniquely in the relation.

There can be, at most, one primary key for a relation.

There can be one or more candidate keys for a relation.

It does not allow null values.

It may contain null values.

It is chosen from one of the candidate keys.

We can find the number of super keys for a relation using candidate keys.

It can be applied as a candidate key.

It may be or may not be employed as a primary key.

What is a Primary Key?

A primary key is chosen from one of the candidate keys. It refers to the attribute or set of attributes uniquely identifying a tuple in the relation. The difference between primary key and candidate key is that a primary key is chosen from one of the candidate keys. The characteristics of a primary key are given below.

  • A primary only allows unique values.
  • It does not allow null values.
  • A relation can have at most one primary key.
  • It is selected from one of the candidate keys.

What is a Candidate Key?

A candidate key is the minimal set of attributes uniquely identifying a tuple in the relation. Using the candidate keys, we can find the number of super keys for a relationship. A relation can have many candidate keys. Following are the characteristics of a candidate key.

  • A candidate key may allow null values.
  • There can be one or more candidate keys for a relation.
  • It only allows unique + null values.

Note: The primary key is chosen from one of the candidate keys with unique and no null values.

Key Differences Between Primary Key and Candidate Key

The difference between primary key and candidate key is given below.

  • The primary difference between the two keys is that a table or relation in a schema can have more than one candidate key but only one primary key.
  • Although defining a primary key is not required, a connection cannot exist without the candidate key.
  • A table or relation's primary key is a distinct and important characteristic. On the other hand, the candidate keys have many candidates, from which one candidate key can be chosen as the primary key.

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FAQs on Difference Between Primary Key and Candidate Key

  • The difference between primary key and candidate key is that a record in a table is uniquely identified by its primary key, which must be both unique and non-null whereas a candidate key can be used in conjunction with a primary key to uniquely identify records in a table.

  • Yes, a primary key is selected from one of the candidate keys by the database administrator to uniquely identify a tuple or record in the relation. The candidate key that contains only unique and non-null values is selected to be the primary key.

  • Based on their usage, the difference between primary key and candidate key is that a candidate key may or may not be used as a primary key while a primary key can be used as a candidate key.

  • A primary key is defined as the set of attributes that identifies rows uniquely in the relation. A primary key only allows unique and non-null values. So it never permits a null value. However, this adds to the difference between primary and candidate keys, as candidate keys can have null values.

  • The terms primary key and candidate key are interchangeable. All potential keys have the same objective, which is to protect data integrity by avoiding data duplication. If it's required to prevent some data from being duplicated, it makes perfect sense for a table to have more than one candidate key enforced in it.

  • The primary difference between the two keys is that a table can have more than one candidate key but only one primary key. Although defining a primary key is not required, a relationship cannot exist without the candidate key.

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