Foreign Key in SQL – Maintaining Relationships Between Tables

Introduction

In SQL, text data types are used to store alphanumeric values like names, addresses, emails, and descriptions. Choosing the correct text type — CHAR, VARCHAR, or TEXT — is important for optimizing storage space, query speed, and database performance.

In this section, you'll learn the definitions, differences, and best use cases for each text data type.

1. CHAR (Fixed-Length String)

CHAR is used to store fixed-length strings. If the stored string is shorter than the defined length, SQL automatically pads it with spaces to match the specified size.

Features:

• Fixed length
• Fast and predictable performance
• Uses extra storage if the data is often shorter than the specified length

Syntax:

column_name CHAR(length);

length = number of characters (1 to 255 depending on the database system)

Example:

CREATE TABLE countries (
country_code CHAR(2),
country_name CHAR(50)
    );

country_code like 'US', 'IN', 'UK' will always take 2 characters.

When to Use CHAR:

• Data with a constant size, such as country codes, gender ('M', 'F'), state abbreviations
• Fixed-format fields like credit card types ('VISA', 'MC')
• When exact storage size is known and consistent

2. VARCHAR (Variable-Length String)

VARCHAR stands for Variable Character. It stores variable-length strings, meaning only the actual characters are stored without unnecessary padding.

Features:

• Variable length
• More space-efficient than CHAR for varying-length text
• Slightly slower than CHAR when processing large volumes (because of extra calculations for string lengths)

Syntax:

column_name VARCHAR(length);

length = maximum number of characters allowed

Example:

CREATE TABLE employees (
first_name VARCHAR(50),
email VARCHAR(100)
    );

Names and emails can vary in length, making VARCHAR ideal.

When to Use VARCHAR:

• Data with unpredictable or variable length
• Names, emails, addresses, and descriptions under 255-65535 characters
• Most general-purpose text fields

3. TEXT (Large Text Field)

TEXT is used to store large amounts of text like long descriptions, blog posts, comments, or articles.

Features:

• Meant for large text storage (up to 65,535 characters for standard TEXT in MySQL)
• Cannot have a default value (in some databases like MySQL)
• TEXT fields are stored outside the main table with a pointer reference
• Different variants exist (TINYTEXT, MEDIUMTEXT, LONGTEXT) for various sizes

Syntax:

column_name TEXT;

Example:

CREATE TABLE articles (
id INT,
title VARCHAR(255),
body TEXT
   );

body will store the full article content, which can be very large.

When to Use TEXT:

• Long-form text fields (comments, articles, reviews, reports)
• Data that exceeds normal VARCHAR limits
• When exact storage requirements are unknown or potentially very large

Quick Comparison: CHAR vs VARCHAR vs TEXT

Important Tips

• Use CHAR only when all values will be exactly the same length
• VARCHAR is the best choice for most standard text fields
• Reserve TEXT for content that exceeds VARCHAR limits
• Consider VARCHAR(MAX) in SQL Server for large text that might need indexing
• Be aware that TEXT fields may have limitations on default values and full-text indexing

What Is a Foreign Key?

A foreign key is a column (or group of columns) in one table that refers to the primary key in another table. It is used to establish a relationship between the two tables and ensures referential integrity.

In simple terms, a foreign key connects one table's data to another — like linking orders to the customers who placed them.

Why Use Foreign Keys?

• Enforces data consistency between related tables
• Prevents invalid data entries (e.g., an order for a non-existent customer)
• Helps define parent-child relationships in relational databases
• Automatically restricts or cascades updates and deletions

Syntax to Create a Foreign Key

1. Creating Foreign Key in Table Definition

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

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

Here, orders.customer_id is a foreign key that references customers.customer_id.

2. Adding a Foreign Key to an Existing Table

ALTER TABLE orders
ADD CONSTRAINT fk_customer
FOREIGN KEY (customer_id) REFERENCES customers(customer_id);

You can also give the foreign key constraint a custom name like fk_customer.

ON DELETE and ON UPDATE Options

You can define actions when the referenced (parent) row is updated or deleted:

FOREIGN KEY (customer_id)
REFERENCES customers(customer_id)
ON DELETE CASCADE
ON UPDATE CASCADE;

Common options:

Example: Enforcing Valid Relationships

-- This will fail if customer_id 999 doesn't exist in customers
INSERT INTO orders (order_id, customer_id, order_date)
VALUES (101, 999, '2025-05-09');

The foreign key constraint will prevent invalid references to non-existent customers.

Foreign Key Naming Best Practices

• Use descriptive names like fk_orders_customer_id
• Keep naming consistent across tables for readability
• Use INFORMATION_SCHEMA.TABLE_CONSTRAINTS to review keys in your DBMS