Candidate Key vs. Primary Key

Attribute	Candidate Key	Primary Key
Definition	A candidate key is a set of attributes that can uniquely identify a tuple in a relation.	A primary key is a candidate key that is chosen to uniquely identify tuples in a relation.
Uniqueness	Each candidate key is unique within a relation.	A primary key is unique within a relation.
Nullability	A candidate key can contain null values.	A primary key cannot contain null values.
Number of Keys	A relation can have multiple candidate keys.	A relation can have only one primary key.
Selection	Any candidate key can be selected as a primary key.	One candidate key is selected as the primary key.
Usage	Candidate keys are used to identify tuples and establish relationships between tables.	Primary keys are used to uniquely identify tuples and enforce entity integrity.

Introduction

In the world of databases, keys play a crucial role in ensuring data integrity and facilitating efficient data retrieval. Two commonly used types of keys are the candidate key and the primary key. While both serve similar purposes, they have distinct attributes that set them apart. In this article, we will explore the characteristics of candidate keys and primary keys, highlighting their similarities and differences.

Candidate Key

A candidate key is a set of attributes within a relation that can uniquely identify each tuple or row in a database table. It is important to note that a relation can have multiple candidate keys, and each candidate key must be unique and minimal. By minimal, we mean that no subset of the candidate key can uniquely identify the tuples in the relation.

For example, consider a table called "Employees" with attributes such as EmployeeID, SocialSecurityNumber, and Email. In this case, both EmployeeID and SocialSecurityNumber can be considered candidate keys since they can uniquely identify each employee. However, if we remove either attribute from the set, it would no longer be a candidate key as it would not satisfy the uniqueness requirement.

One of the advantages of using candidate keys is their flexibility. They allow for multiple options when choosing a primary key, which can be beneficial in certain scenarios. Additionally, candidate keys provide a way to ensure data integrity by enforcing uniqueness constraints on the table.

However, candidate keys also have some limitations. The presence of multiple candidate keys can increase the complexity of the database design and query optimization. It requires careful consideration to select the most appropriate candidate key as the primary key, taking into account factors such as data access patterns and performance requirements.

Primary Key

A primary key is a specific candidate key chosen from the set of candidate keys to uniquely identify each tuple in a relation. It serves as a unique identifier for the records in a table and is used to establish relationships between tables in a relational database. Unlike candidate keys, a relation can have only one primary key.

When selecting a primary key, it is essential to choose a candidate key that best represents the entity being modeled and is stable over time. Stability ensures that the primary key value does not change frequently, as it would require updating all related records in other tables, leading to potential data inconsistencies.

For instance, in the "Employees" table mentioned earlier, if we choose EmployeeID as the primary key, it would uniquely identify each employee and remain stable over time. This stability is crucial as it allows other tables to establish relationships with the "Employees" table based on the primary key.

One of the significant advantages of using a primary key is its simplicity. It provides a clear and straightforward way to identify and retrieve specific records from a table. Additionally, primary keys are often used as foreign keys in other tables to establish referential integrity, ensuring consistency and accuracy in the database.

However, primary keys also have some limitations. The choice of a primary key is permanent and cannot be easily changed once established. This can pose challenges if the selected primary key becomes inadequate or if there is a need to change the primary key due to evolving business requirements. Additionally, primary keys can introduce additional storage overhead, especially when used as foreign keys in related tables.

Similarities

While candidate keys and primary keys have distinct attributes, they also share some similarities. Both candidate keys and primary keys are used to uniquely identify tuples in a relation, ensuring data integrity and consistency. They both enforce uniqueness constraints on the table, preventing duplicate records from being inserted. Additionally, both candidate keys and primary keys can be used as references in relationships between tables, establishing the foundation for relational databases.

Differences

Although candidate keys and primary keys have similarities, they differ in several aspects. One of the key differences is that a relation can have multiple candidate keys, but only one primary key. Candidate keys provide flexibility in choosing the primary key, while the primary key is a specific candidate key chosen to uniquely identify tuples.

Another difference lies in the stability of the keys. Candidate keys may change over time, while the primary key should ideally remain stable to maintain referential integrity. Changing a primary key can have cascading effects on related records, making it a more critical decision compared to changing a candidate key.

Furthermore, the selection process for candidate keys and primary keys differs. Candidate keys can be chosen based on various factors, such as uniqueness and minimalism, while the primary key selection requires additional considerations, such as stability and suitability for establishing relationships.

Lastly, the usage of candidate keys and primary keys in database design and query optimization also differs. The presence of multiple candidate keys can impact the complexity of the design and optimization process, while the primary key provides a straightforward and efficient way to identify and retrieve records.

Conclusion

In summary, candidate keys and primary keys are essential components of relational databases. While both serve the purpose of uniquely identifying tuples, they have distinct attributes that set them apart. Candidate keys offer flexibility and enforce uniqueness constraints, while primary keys provide stability and simplicity in data retrieval. Understanding the similarities and differences between candidate keys and primary keys is crucial for designing efficient and robust database systems.