DBMS: Key Differences Between Primary Key And Candidate Key

The surge in data usage has given rise to the need for structured and efficient management. This is where Database Management Systems (DBMS) step in. At the heart of DBMS lies the concept of keys – indispensable tools that unlock the potential of organized data storage and retrieval. Key in the database is very useful while designing the database schema, and understanding and optimizing database schema as well. In this article, we will look at the difference between candidate key and primary key, two keys that are important while creating relations between tables. But before that, let's get started with a few basics!

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What Is Key?

A key in Database Management System, or DBMS, is an attribute or name of the column from the value of which we can identify records(or rows) in the database table or database relation. e.g., super key, foreign key, etc.

Below are the keys that are used in DBMS, each key has different functionalities and use cases:

Primary Key
Foreign Key
Candidate Key
Super Key
Composite Key
Compound Key
Surrogate Key

In this article, we are going to understand about primary key and candidate key and key differences between them. This article will be helpful for preparing for interview questions, as well as for your college examinations.

What Is Primary Key?

Definition

Primary Key is a collection of attributes or a single attribute that has the capability to uniquely identify records in the table. The attribute is a characteristic of an entity or table. In general, we can say that we can find the required record(or row) from the table with the help of a primary key.

The primary key is basically a minimal super key. That's why One table can have only one primary key. The primary key is unique and not null as it provides individual uniqueness. This means that the attribute which is selected as the primary key should have a different value for every record and it should not have any null value.

Example

To understand the primary key with an example, consider below Employee table:

Employee_Id	Employee_Name	Employee_Designation	Employee_Salary
1001	Aryan	CEO	250000
1002	Anuj	CTO	200000
1003	Sachin	Project Manager	150000
1004	Prince	Project Manager	150000

This table illustrates 4 attributes Employee_Id, Employee_name, Employee_Deisgnation, and Employee_Salary. Here as you can see Employee name can be the same for two employees, a designation also cannot be unique and the salary of the employees can also be the same. In the above table, only Employee_id can be used as the primary key.

What Is Candidate Key?

Definition

Candidate Key is a collection of attributes or a single attribute(single column) that has the ability to uniquely identify records in the table. Now confusion between the primary key and the candidate key arises. As the primary key is a minimal super key, one database table can have only one primary key but one table can have more than one candidate key.

Candidate key attribute can contain null values, but it does not allow the same value in the attribute field as its definition says that it uniquely identifies the record(or tuple) in the field. Also, Candidate keys cannot have redundant attributes as in the super keys, or in other words, the candidate key should contain minimum fields.

The type of candidate key which is chosen as the primary key will be the primary key in the table. This is the essential difference between them.

The columns that participate in the formation of candidate keys, primary keys, or any other keys that type of columns are called Key columns.

Example

The below table of Student Records illustrates the purpose of the candidate key.

Id No.	Name	Email-id	Department
001	Aryan	aryan12@gmail.com	IT
002	Prince	prince1@gmail.com	CE
003	Sachin	null	EC
004	Virat	virat12@gmail.com	CE

The above table contains 4 attributes, Id No, Name, Email-id, and Department of Student. Among these two attributes Id No. and Email-Id can both be considered candidate keys. Because both attributes have the ability to uniquely identify records or rows in the table.

But here noticeable thing is that the email-id of Sachin is not present, still, it can be considered as a candidate key. And among these two attributes, one will become a primary key of the table. By looking at both candidate keys, it is obvious that Id No. will become a primary key of this table because email-id contains the null value and primary is the type of key that does not allow null values.

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Differences Between Primary Key And Candidate Key

This is the main part of the article after understanding both the primary key and the candidate key. The below table demonstrates the key differences between both keys.

Primary Key	Candidate Key
The primary key uniquely identifies a record or row in the table, and it follows one key per table approach.	The candidate key also uniquely identifies a record or row in the table, but it can have one key per table or many keys per table.
The primary key is a minimal super key.	The candidate key is not a minimal super key.
For the attribute which is selected as the primary key, it will always have unique and non-null values.	For the attribute which is selected as a candidate key, it will always have a unique value. It may contain null values unless the attribute constraint is specified as not null.
The primary key for the table is chosen from a set of candidate keys.	The candidate key for the table is chosen from the super key.
Poor choice of the primary key can lead to unsuccessful relations between tables, and eventually it complexes the whole database schema.	The candidate key does not have choices.
When the relationship between one table is created, the primary key in one table will become a foreign key in another table.	The candidate key does not have much of a role while creating relationships between tables.

Frequently Asked Questions

Q1. What is the Role of Keys in Database Design?

Keys are essential in uniquely identifying records within a database. They ensure data integrity, facilitate relationships between tables, and optimize data retrieval.

Q2. What is a Primary Key?

A primary key is a unique identifier for each record in a table. It differentiates records, maintains data accuracy, and plays a crucial role in establishing relationships between tables.

Q3. Can Primary Key Values be Duplicated?

No, primary key values must be unique for each record in a table. This uniqueness is essential for preventing data duplication and maintaining accurate identification.

Q4. What is the Difference Between a Primary Key and a Candidate Key?

A primary key is the selected unique identifier for records within a table. In contrast, a candidate key is a potential option for a primary key and possesses the same attributes, including uniqueness and non-nullability.

Q5. Can a Table Have Multiple Candidate Keys?

Yes, a table can have multiple candidate keys. These are potential choices for the primary key, with each candidate key representing a unique identification option.

Q6. Can a Candidate Key Become a Primary Key?

es, one candidate key can be chosen as the primary key for a table. The remaining candidate keys, though not selected as primary, still serve as alternate means of uniquely identifying records.

Q7. Why are Primary Keys Important for Relationships Between Tables?

Primary keys are crucial for establishing relationships between tables. They act as references in related tables, ensuring data consistency and accurate linkage.

Q0. Are Candidate Keys Necessary for Every Table?

While not every table requires multiple candidate keys, considering potential candidate keys during database design is beneficial. It allows for flexibility and adaptation to changing requirements.

In summation, we can say that the primary key has to be chosen carefully from various candidate keys available, and eventually, it will be helpful for database creation and building relationships among tables. We hope you found this article helpful. Happy Learning👋.

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Shreeya Thakur

Sr. Associate Content Writer at Unstop

I am a biotechnologist-turned-content writer and try to add an element of science in my writings wherever possible. Apart from writing, I like to cook, read and travel.