Difference between Primary and Candidate Key (original) (raw)

Last Updated : 11 Jul, 2025

In relational database management systems(RDBMS) both the **Primary Key and **Candidate Key are the essential components and are used to uniquely identify records (tuples) within a table. They both are fundamental concepts used to ensure data integrity and prevent duplication of data. These(Primary key and Candidate key) are also can be used to create a relationship between two tables. Understanding the difference between them is essential for building robust and optimized database systems.

**What is Primary Key?

Primary Key is a set of attributes(s) that uniquely identify the tuples in relation or table. The primary key is a minimal super key, so **there is one and only one primary key in any relationship. A Primary key is unique to ensure that each record in the table is distinct and easily identifiable. For example,

**Student{ID, Aadhar_ID, F_name, M_name, L_name, Age}

Here only **ID or **Aadhar_ID can be the primary key because the name and age can be the same, but ID or Aadhar_ID can't be the same.

Advantages of the primary key

**Easy to search: we were able to find the data quickly of anything using their Unique ID or primary key, especially for the larger datasets.
**No duplicates: It ensures that each record has its unique ID. In the above example, StudentID is telling that each student is unique from others even if their name and surname and any record match and are confusing.
**Automatically Faster: Query Optimization becomes automatically faster because of indexing and the database also makes searching faster automatically with the help of the primary key.

Disadvantage of the Primary Key

**No NULL values: It cannot contain any NULL values, and in some scenarios this may limit flexibility.
**Single key: we can only have one primary key in a table, so because of this other potential unique attributes cannot able to become primary.

**What is Candidate Key?

A candidate key is a set of attribute(s) that uniquely identify the tuples in relation or table. As we know the Primary key is a minimal super key, so there is one and only one primary key in any relationship but there is more than one candidate key that can take place. The candidate key's attributes can contain a NULL value that opposes the primary key. **For example,

Student{ID, Aadhar_ID, F_name, M_name, L_name, Age}

Here we can see the two candidate keys **ID and **Aadhar_ID. So there is more than one candidate key, which can uniquely identify a tuple in a relation or able to become the primary key.

Advantages of Candidate Key

**Flexibility: There are chances of having multiple candidate in a table which provides flexibility in choosing the one primary key that suits the best for the table among all candidate key.
**Uniqueness: It ensures that even if the primary key is not suitable still the records can be identified uniquely.
**Backup for primary key: If there is any need or primary key is not available, then candidate keys can be used as a backups for the primary key.

Disadvantages of Candidate Key

**More Complex: H aving multiple candidate key can make the table complex to manage and understanding the database schema as well.
**Extra Space: If multiple candidate keys are indexed, then they can add an extra storage and indexing overhead.

Difference Between Primary and Candidate Key

Primary Key	Candidate Key
The primary key is a minimal super key. So there is one and only one primary key in a relation.	While in a relation there can be more than one candidate key.
Any attribute of the Primary key can not contain a NULL value.	While in the Candidate key, any attribute can contain a NULL value.
The primary key can be optional to specify any relation.	But without the candidate key, there can't be specified any relation.
The primary key specifies the important attribute for the relation.	The candidate specifies the key which can qualify for the primary key.
It's confirmed that a primary key is a candidate key.	But It's not confirmed that a candidate key can be a primary key
It is unique for every record and ensures no duplicate data.	There can be multiple candidate keys in a table which can be unique but among them only one becomes primary.
For optimizing the performance of the query the primary key is indexed by default.	The candidate key may not be indexed unless specified.
It takes less storage space since there is only one primary key exists for a table.	May take more storage if indexed, as there can be multiple candidates present in a table.
A primary key cannot contain composite attributes(attributes that can be subdivided into smaller attributes).	Candidate keys can be composed of composite attributes, which depend on the structure of tables.

Conclusion

In summary, both the primary and candidate keys serve the similar purpose of uniquely identifying records from the tables, the primary key is the one that is chosen to enforce entity integrity in a database and ensures that there are no duplicates or NULL values in key columns. Candidate keys, on the contrary, provide flexibility by offering multiple options for what could be the primary key or which one column will be able to become the primary key from the columns list and the rest remains to be candidate key. Understanding their differences helps in designing an efficient and robust database structure.