A foreign key is a column or set of columns in a table that refers to the primary key of another table. It is used to establish a relationship between two tables.

When we study databases, one important idea is how different tables connect to each other.
Just like in real life a student belongs to a class, or an employee belongs to a department, database tables also have relationships.
To maintain these relationships in a clear and reliable way, SQL uses something called a foreign key.

A foreign key is a column in one table that refers to the primary key of another table.
It acts like a link or a bridge that connects two tables together, ensuring that the relationship between them stays accurate.

I often explain this to my students using a simple analogy:
If the primary key is a person’s unique identity, then the foreign key is like a reference to that identity in another list.
It tells us “this row belongs to that record in the other table.”

Key points about a foreign key:

• It establishes a relationship between two tables.
• It ensures data integrity—you cannot enter a value that doesn’t exist in the parent table.
• It prevents accidental deletion of important linked data.
• It helps maintain consistency across related tables.

Simple Example

Imagine two tables:

1. Departments Table

2. Employees Table

Here, DeptID in the Employees table is a foreign key pointing to
DeptID in the Departments table.

That means:

• Employee Aditi works in the Finance department
• Rohan works in IT
• Meera works in HR

SQL will not allow you to enter a DeptID like “50” in Employees, because no such department exists in the Departments table.

Why is a foreign key important?

A foreign key ensures:

• Data remains logically connected
• No invalid values are inserted
• Related records stay consistent
• The database mirrors real-world relationships correctly

Without foreign keys, databases can quickly become messy, full of mismatched or orphaned records.