← Back to Home
🔷 Part 4: Primary Keys and Foreign Keys – Building Relationships in Databases
📍 Introduction
So far, we’ve learned how data is stored in tables and how we use SQL to interact with it. But what happens when your database has more than one table?
That’s where relationships come in — and they’re built using keys.
Keys help connect tables, maintain data integrity, and allow complex queries across multiple data sets. Today, we’ll explore Primary Keys and Foreign Keys, the foundation of relational database design.
🔑 What is a Primary Key?
A Primary Key is a unique identifier for each record in a table. No two rows can have the same primary key, and it can’t be left empty (NULL).
🔍 Example – Students Table:
| StudentID | Name | Course |
|-----------|-------|------------|
| 1 | Aisha | Math |
| 2 | Ravi | Science |
| 3 | Sara | English |
Here, StudentID is the Primary Key — each student has a unique ID.
📘 Think of it like a roll number in a classroom — no two students have the same roll number.
🔗 What is a Foreign Key?
A Foreign Key is a column in one table that refers to the primary key in another table. It creates a link between two related tables.
🧩 Example: Students & Results Tables
🧾 Students Table
| StudentID | Name |
|-----------|-------|
| 1 | Aisha |
| 2 | Ravi |
🧾 Results Table
| ResultID | StudentID | Subject | Marks |
|----------|-----------|----------|-------|
| 101 | 1 | Math | 85 |
| 102 | 2 | Science | 90 |
In this case:
-
StudentIDis the Primary Key in the Students table. -
StudentIDin the Results table is a Foreign Key — it refers to the Students table.
💡 This relationship ensures that every result belongs to a valid student.
🔁 Why Use Keys and Relationships?
-
✅ Data Integrity: Prevents orphaned or mismatched records
-
✅ Less Redundancy: No need to repeat data across tables
-
✅ Scalability: Makes your database easier to maintain as it grows
-
✅ Real-Life Modeling: Mimics real-world relationships (students have results, customers place orders, etc.)
🔧 SQL Example: Defining Primary and Foreign Keys
-- Create Students Table
CREATE TABLE Students (
StudentID INT PRIMARY KEY,
Name VARCHAR(100)
);
-- Create Results Table
CREATE TABLE Results (
ResultID INT PRIMARY KEY,
StudentID INT,
Subject VARCHAR(50),
Marks INT,
FOREIGN KEY (StudentID) REFERENCES Students(StudentID)
);
📚 Real-Life Analogy
Imagine:
-
Primary Key = National ID Number
-
Foreign Key = Form asking for your National ID
You can’t submit the form unless your ID is valid — just like foreign keys rely on real, matching primary keys.
🧠 Recap
-
Primary Key: Uniquely identifies a row in a table (e.g., StudentID)
-
Foreign Key: Connects a row to another table using a reference
-
These keys help create relationships between tables and ensure the accuracy and reliability of your data.
✅ What’s Next?
In Part 5, we’ll explore NoSQL databases — a modern alternative to relational databases that works better for unstructured data, large-scale applications, and real-time needs.