Difference Between Candidate Key and Primary Key

Understanding the differences between Candidate Keys and Primary Keys is crucial in the field of database management. A Candidate Key is a set of attributes uniquely identifying rows in a table, and there can be multiple candidate keys. On the other hand, the Primary Key is a special candidate key selected to identify each record in the table uniquely. This article delves into the subtleties of these two types of keys, exploring their characteristics, constraints, and their critical roles in database architecture. 

In database management, keys are vital in ensuring data integrity and facilitating efficient retrieval and manipulation of information. Two key concepts in this domain are the candidate and primary keys. While closely related, they serve distinct purposes and exhibit certain differences. This article highlights the difference between candidate and primary keys, clearly understanding their definitions, examples, and comparative features.

Differences Between Candidate Key and Primary Key

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What is a Candidate Key?

A candidate key is a collection of attributes or a single attribute (single column) uniquely identifying a table record. A primary key is a minimal superkey, so that a database table can have only one primary key, but a table can have multiple candidate keys.

In this example, several attributes have the potential to identify each student in the table uniquely. These attributes are candidate keys. Let’s consider two candidate keys: “StudentID” and “Email”.

Candidate Key 1: StudentID The “StudentID” attribute uniquely identifies each student in the table. It satisfies the requirements of uniqueness and minimality. No two students share the same StudentID in this table.

Candidate Key 2: Email The “Email” attribute also has the potential to identify each student uniquely. It satisfies the requirements of uniqueness and minimality. No two students have the same email address in this table.

Both “StudentID” and “Email” are candidate keys in this example because they can uniquely identify each student. They provide different ways to identify a student within the table uniquely.

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What is the Primary Key?

A primary key is a minimal super key. Because of this, we can only have one primary key on the table. The primary key independently provides uniqueness, so it is unique and not null. This means that the attribute chosen as the primary key should have a different value for each record, not a null value. 

Example of Primary Key

Let’s consider an example to understand the primary key concept better. Imagine we have a table called “Employees” that stores information about employees in a company. The table has the following attributes: EmployeeID, Name, Age, and Department.

The “EmployeeID” column is designated as the primary key in this example. It plays a crucial role in uniquely identifying each employee and maintaining the integrity of the data within the “Employees” table. This is because, Here, as you can see, Employee names can be the same for two employees, a department also cannot be unique, and the age of the employees can also be the same. Only Employee_id can be used as the primary key in the above table.

Conclusion

These two database keys are essential to a relational database, providing unique identification and maintaining data integrity. While candidate keys serve as potential choices for primary keys, primary keys are selected from among the candidate keys to identify each row in a table uniquely. Understanding the differences between these key types is crucial for designing effective and efficient database systems.

Remember, candidate keys play a significant role in determining the primary key, which is the cornerstone of data integrity and efficient retrieval within a database.