Primary Key and Foreign Key in SQL

Understanding the significance of keys is crucial in relational database management to maintain table relationships and guarantee data uniqueness. The main key serves as a foundation, guaranteeing that each database entry is uniquely identified. By connecting to the primary key of another table, on the other hand, the Foreign Key is essential in creating connections across tables. A well-structured database is built on this complex relationship. In this blog post, “Primary Key and Foreign Key in SQL,” we will delve into the details of these keys and their responsibilities in building a strong, connected database. For those eager to enhance their database management skills, especially in MySQL, consider MySQL training in Chennai.

What is the Primary Key in SQL?

In SQL, a Primary Key is a field or a set of fields in a database table that uniquely identifies each record in that table. It ensures the uniqueness of each entry, serving as a distinctive identifier. A primary key is crucial for maintaining data integrity, as it prevents the insertion of duplicate or null values in the key column. The primary key is typically selected from existing columns in the table and plays a pivotal role in establishing relationships with other tables in the database. It serves as a fundamental component of database design in SQL, facilitating efficient data retrieval and management.

How Primary Keys Work in SQL:

  • Uniqueness: Primary keys ensure that each record in a database table has a unique identifier, preventing duplication.
  • Non-null Values: The primary key column cannot have null values, ensuring that every record has a valid identifier.
  • Table Relationships: Primary keys establish relationships between tables. Other tables can link a column (foreign key) to the primary key, connecting data.
  • Indexing: Primary keys are often automatically indexed for faster data retrieval during queries.
  • Database Integrity: They play a crucial role in maintaining the integrity of the database by preventing duplicate entries.
  • Selection and Constraints: Primary keys can be a single column or a combination. SQL allows the definition of constraints for uniqueness and non-null requirements.

In SQL, Primary Keys form the backbone of database structure, ensuring each record is uniquely identified, relationships are established, and data integrity is maintained.

Primary Key in SQL with example

In the context of SQL, a Primary Key is a column or set of columns in a table that uniquely identifies each record. Let’s illustrate this concept with a different example:

Imagine a table called “Products” with the following structure:

CREATE TABLE Products ( ProductCode VARCHAR(10) PRIMARY KEY, ProductName VARCHAR(100), Price DECIMAL(10, 2), StockQuantity INT );

In this example:

  • ProductCode serves as the primary key, defined as a unique identifier for each product and of type VARCHAR(10).
  • Additional columns include ProductName for the product name, Price for the product’s price, and StockQuantity for the available stock.

Let’s insert some sample data:

INSERT INTO Products (ProductCode, ProductName, Price, StockQuantity) VALUES (‘P001’, ‘Laptop’, 899.99, 50), (‘P002’, ‘Smartphone’, 499.99, 100), (‘P003’, ‘Tablet’, 299.99, 75);

Here, the ProductCode column acts as the primary key, ensuring each product has a distinct identifier. Any attempt to insert a record with a duplicate ProductCode or a null value will be rejected, maintaining data uniqueness and integrity.

This primary key can be referenced in other tables as a foreign key, creating relationships between tables and facilitating a well-organized relational database structure.

What is Foreign Key in SQL?

In SQL, a Foreign Key is a field or a set of fields in a database table that establishes a link to the Primary Key of another table. It creates a relationship between the data in two tables, enforcing referential integrity. A foreign key ensures that values in the foreign key column of one table correspond to the values in the primary key column of another table, connecting related data. This key plays a crucial role in maintaining the consistency of relationships between tables and preventing the insertion of invalid data. Just like the primary key, the foreign key is integral to establishing structured and interconnected relationships in SQL database design.

How Foreign Keys Work in SQL:

  • Establishing Relationships: Foreign keys create links between tables by referencing the primary key of another table. This establishes relationships, ensuring data consistency.
  • Referential Integrity: Foreign keys enforce referential integrity by ensuring that values in the foreign key column correspond to valid entries in the referenced primary key column.
  • Preventing Orphaned Records: Foreign keys prevent the creation of orphaned records by requiring that each entry in the foreign key column has a corresponding entry in the referenced primary key column.
  • Cascading Actions: SQL allows defining actions to be taken when a referenced record is modified or deleted. Common actions include cascading updates or deletes to maintain consistency.
  • Enhancing Data Integrity: By linking tables through foreign keys, SQL enhances data integrity across related tables, reducing the risk of inconsistencies or errors.
  • Structured Database Design: Foreign keys contribute to a well-structured database design, facilitating organized relationships between tables for efficient data management.

In essence, foreign keys in SQL serve as crucial components for establishing and maintaining relationships between tables, ensuring data accuracy, and promoting a structured and interconnected database design. To further enhance your understanding of database optimization and performance, it’s valuable to explore the various types of SQL Server Indexes.

Foreign Key in SQL with example

In SQL, a Foreign Key is a column or set of columns in a table that establishes a connection to the Primary Key of another table. Let’s illustrate this concept with a distinct example:

Consider two tables, “Customers” and “Orders.” The “Orders” table might include a Foreign Key referencing the “CustomerID” column in the “Customers” table.

Here’s an example SQL script:

— Creating Customers table with a Primary Key

CREATE TABLE Customers (


CustomerName VARCHAR(50),

Email VARCHAR(50)


— Creating Orders table with a Foreign Key referencing Customers table



OrderDate DATE,

CustomerID INT,

FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID)


In this example:

The “Customers” table has a Primary Key on the “CustomerID” column.

The “Orders” table has a Foreign Key on the “CustomerID” column, referencing the Primary Key in the “Customers” table.

Let’s insert some data:

— Inserting data into Customers table

INSERT INTO Customers (CustomerID, CustomerName, Email)

VALUES (1, Shree Kumar, [email protected]’),

    (2, Akash Singh, ‘[email protected]’);

— Inserting data into Orders table with valid CustomerID references

INSERT INTO Orders (OrderID, OrderDate, CustomerID)

VALUES (101, ‘2023-01-15’, 1),

    (102, ‘2023-02-20’, 2);

In this scenario:

The “Orders” table’s “CustomerID” column references the “CustomerID” column in the “Customers” table.

Each order in the “Orders” table must have a valid “CustomerID” that corresponds to an existing customer in the “Customers” table. This demonstrates how Foreign Keys establish relationships between tables, ensuring referential integrity and preventing the insertion of invalid data.


In the world of SQL databases, the Primary Key and Foreign Key are essential elements that ensure data accuracy and enable meaningful connections between tables. The Primary Key acts as a unique identifier a table, preventing data duplication and serving as a crucial foundation for relationships. Meanwhile, the Foreign Key establishes links between tables by referencing the Primary Key of another table. 

Together, these keys form the backbone of well-organized and interconnected databases, facilitating effective data management. In essence, the thoughtful use of Primary and Foreign Keys is fundamental for creating reliable and robust SQL databases. SQL Training in Chennai is a valuable resource for anyone looking to master the intricacies of SQL databases and advance their career in database management.