Clustered vs Non-Clustered Index in SQL – Key Differences and Examples

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 Clustered Index?

A clustered index determines the physical order of data in the table. Each table can have only one clustered index, because the data rows themselves are stored in that order.

Think of it like a sorted phonebook — the data is physically arranged by last name.

Key Points:

• Data is sorted and stored based on the index
• Only one clustered index per table
• Typically created on the primary key
• Improves performance for range-based queries

Example:

CREATE CLUSTERED INDEX idx_employee_id
ON employees (employee_id);

This will physically sort the employees table by employee_id.

What Is a Non-Clustered Index?

A non-clustered index creates a separate structure that points to the actual data rows. It doesn't affect the table's physical order.

Think of it like an index at the back of a book that points you to a specific page.

Key Points:

• Data remains unsorted
• Can have multiple non-clustered indexes per table
• Contains pointers to actual rows (row locators)
• Great for speeding up lookups and filters on non-key columns

Example:

CREATE NONCLUSTERED INDEX idx_last_name
ON employees (last_name);

This index helps find rows with a specific last_name efficiently, without changing the physical order.

Clustered vs Non-Clustered Index – Comparison Table

Pro Tip:

In many databases (e.g., SQL Server, PostgreSQL), the primary key is clustered by default unless specified otherwise.

When to Use Which?

Use a clustered index for:

• Primary keys
• Columns often used in ORDER BY
• Range queries (e.g., date ranges)

Use a non-clustered index for:

• Frequently searched columns
• Columns used in JOIN, WHERE, or GROUP BY
• Supporting multiple access paths