Learning SQL: PRIMARY VS FOREGIN KEYS

What Are PRIMARY and FOREIGN Keys?

In SQL, PRIMARY KEY and FOREIGN KEY are essential concepts for defining relationships between tables and ensuring data integrity.

• A PRIMARY KEY is a column (or set of columns) that uniquely identifies each row in a table.
• A FOREIGN KEY is a column that creates a link between two tables by referencing the PRIMARY KEY of another table.

Think of PRIMARY KEY as a unique identifier (like a Social Security Number), while a FOREIGN KEY establishes a relationship (like a customer ID in an orders table that links back to the customers table).

How Do PRIMARY and FOREIGN Keys Work?

PRIMARY KEY: Ensuring Uniqueness

A PRIMARY KEY ensures that:

• Each row has a unique identifier.
• The column(s) cannot contain NULL values.
• A table can have only one PRIMARY KEY.

Basic Syntax for PRIMARY KEY:

CREATE TABLE customers (
    customer_id INT PRIMARY KEY,
    customer_name VARCHAR(100),
    email VARCHAR(100)
);

Here, customer_id is the PRIMARY KEY, meaning no two customers can have the same ID.

FOREIGN KEY: Linking Tables

A FOREIGN KEY ensures:

• A relationship between two tables (parent and child).
• The value in the FOREIGN KEY column must exist in the referenced PRIMARY KEY column (or be NULL, if allowed).

Basic Syntax for FOREIGN KEY:

CREATE TABLE orders (
    order_id INT PRIMARY KEY,
    customer_id INT,
    order_date DATE,
    amount DECIMAL(10,2),
    FOREIGN KEY (customer_id) REFERENCES customers(customer_id)
);

Here, customer_id in the orders table references customer_id in the customers table, ensuring every order is linked to a valid customer.

Example: How PRIMARY and FOREIGN Keys Work Together

Let’s say we have two tables:

1. customers Table (Parent Table)

   customer_id (PK)	customer_name	email
   1	John Doe	john@example.com
   2	Jane Smith	jane@example.com

2. orders Table (Child Table)

   order_id (PK)	customer_id (FK)	order_date	amount
   101	1	2023-01-15	150.00
   102	2	2023-01-16	200.00

What Happens Here?

• The customer_id in orders must match an existing customer_id in the customers table.
• If you try to insert an order with customer_id = 3 (which doesn’t exist in customers), SQL will reject it to maintain referential integrity.

Key Points to Remember

✅ PRIMARY KEY

• Must be unique and non-NULL.
• Only one per table.
• Can be a single column or a combination (composite key).

✅ FOREIGN KEY

• Ensures data consistency by linking to a PRIMARY KEY.
• Can reference a table in the same or a different database.
• Prevents orphaned records (e.g., an order without a valid customer).

✅ Common Use Cases

• One-to-Many Relationships: One customer can have many orders.
• Many-to-Many Relationships: Requires a junction table with FOREIGN KEYS.

Recap

Feature	PRIMARY KEY	FOREIGN KEY
Purpose	Uniquely identifies a row	Links to a PRIMARY KEY in another table
NULL Values	Not allowed	Allowed (if not set to NOT NULL)
Uniqueness	Must be unique	Can have duplicates (unless constrained)
Number per Table	Only one	Multiple allowed

By understanding PRIMARY and FOREIGN KEYS, you can design efficient, well-structured databases that maintain data integrity.

Resources

PostgreSQL Primary Key

PostgreSQL Foreign Key

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