Structured Query Language (SQL) is a strong and frequently used database management computer language. SQL lets database administrators and developers communicate with databases, construct and manage tables, and conduct other data operations. Primary and foreign keys are crucial concepts to learn while constructing a relational database. These are required for building table associations and guaranteeing data integrity. SQL Training is critical in equipping professionals with the information and abilities to work successfully with primary and foreign keys in database architecture. In this blog, we will look at the Difference Between Primary Keys and Foreign Keys in SQL and their importance in relational database administration.

Table of contents 

  • Understanding Primary Key in SQL 
  • Understanding Foreign Key in SQL 
  • Primary Key vs Foreign Key: Key Differences 
    • Purpose  
    • Individual vs. Referential 
    • Location 
    • Integrity of Data 
    • Alteration 
  • Significance of Primary Key and Foreign Key in SQL 
  • Best Practices for Working with Primary Keys and Foreign Keys in SQL 
  • Conclusion 

What is Primary Key in SQL?

A primary key is an identifier unique to each record (row) in a database. It allows you to uniquely identify and retrieve individual records in the table. Primary keys are critical for preserving data integrity and preventing duplicate records in the table. When constructing a table in SQL, a primary key is established and must fulfil the following rules: 

  1. Each value in the main key column must be unique and cannot be replicated in any other table record. 
  2. The main key column cannot have any null values. The main key column in each record must have a valid and unique value. 
  3. Once a primary key is assigned to a record, its value should not change. This guarantees that the record can be identified uniquely throughout its existence. 

What is Foreign Key in SQL?

A foreign key creates a link or relationship between two tables in a relational database. It denotes a field (or combination of fields) in one database that relates to the primary key in another. Foreign keys are used to establish referential integrity, ensuring consistent data in connected tables. A foreign key is defined in SQL while establishing a table or using ALTER TABLE commands, and it must satisfy the following rules: 

  1. A foreign key value in one table must match a primary key value in another table. If a foreign key refers to a non-existent primary key, the database will either block the operation or cascade modifications as stated. 
  2. A foreign key can sometimes be nullable, enabling the relationship between the tables to be optional. 
  3. Foreign key values, like primary keys, should not change once they are formed. This guarantees that the tables’ link remains maintained. 

Primary Key vs Foreign Key: Key Differences 

Now that we’ve defined main keys and foreign keys let’s look at the key distinctions between the two: 


The primary key’s principal purpose is to uniquely identify each entry in a database, hence assuring data integrity and removing duplicates. 

The fundamental function of the foreign key is to establish a link between two tables by referencing the primary key of another table. 

Individual vs. Referential 

Primary keys are one-of-a-kind and cannot be repeated inside the same table. 

Foreign keys are not need to be unique and can be repeated inside the same database, but they must refer to unique values in the primary key of the associated table. 


The primary key is specified within the table it belongs to, and each table can only have one main key. 

The foreign key is defined in a table that refers to another table’s main key. A table can have numerous foreign keys pointing to a distinct table. 

Integrity of Data 

The primary key ensures data integrity inside the table by guaranteeing that each entry is uniquely identified. 

The foreign key ensures referential integrity between two linked tables by avoiding activities that might violate the relationship or cascade changes in the wrong direction. 


Changing a record’s primary key is not advised since it would need changing all relevant foreign keys in other tables, perhaps resulting in data inconsistency. 

A foreign key value can be changed, but it must be updated in the primary key of the associated table to ensure referential integrity. 

Significance of Primary Key and Foreign Key in SQL 

Correctly using primary and foreign keys is critical for creating efficient and well-structured relational databases. Here’s why they’re necessary: 

  1. Primary keys guarantee that each entry in a database is distinct, eliminating data duplication and preserving table integrity. Foreign keys provide referential integrity by guaranteeing consistent linked data between tables. 
  2. Foreign keys enable significant links across tables, allowing developers to design complicated searches and obtain data from numerous databases. 
  3. Indexes are automatically constructed on primary keys, improving the database’s search and retrieval speed. When combining related tables, foreign keys also benefit from the availability of indexes. 
  4. Database administrators can assure data consistency across connected tables by using foreign keys. This avoids the creation of orphaned records or incorrect relationships. 

Best Practices for Working with Primary Keys and Foreign Keys in SQL 

While primary and foreign keys are valuable tools for guaranteeing data integrity and creating relationships in SQL databases, they must be used per recommended practices. Consider the following recommended practices: 

  1. When picking primary keys, choose meaningful and relevant qualities to each record. Avoid using arbitrary IDs or auto-incremented numbers if more descriptive features are available. 
  2. Primary keys should be simple and not composite keys (made of numerous properties) unless absolutely essential. Simple primary keys are simpler to handle and keep up with. 
  3. Using integer data types for primary keys (such as INT or BIGINT) is best. Integer types are more efficient for indexing and allow quicker data retrieval. 
  4. Once a primary key is established, it should not be changed unless required. Changing primary keys might cause errors in data and cascade changes in connected tables. 


Finally, understanding the difference between primary and foreign keys is crucial for developing well-designed and robust SQL relational databases. A primary key is a unique identifier for each database record that helps ensure data integrity. A foreign key, on the other hand, creates cross-table links that allow for meaningful relationships and data consistency. SQL training equips professionals with the knowledge and skills to effectively utilise primary and foreign keys, ensuring optimal database architecture and maintenance.