What Is an Index in SQL? – Purpose, Types, 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


Feature CHAR VARCHAR TEXT
Storage Fixed length Variable length Variable, large storage
Max Size Up to 255 chars 65,535 bytes (typically) 65,535+ chars (depends on type)
Performance Fast for fixed-size Efficient for variable text Slightly slower for queries
Indexing Full index support Full index support Limited in some DBs
Best Use Case Codes, fixed formats Names, addresses, emails Articles, long descriptions



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


SQL Indexes

Definition: What Is an Index?

An index in SQL is a performance optimization feature that allows the database to find and retrieve data faster, much like an index in a book.

Instead of scanning every row in a table, the database uses the index to locate data quickly, especially in large datasets.

Why Use Indexes?

Indexes are mainly used to:

  • Speed up SELECT queries
  • Improve JOIN performance
  • Accelerate WHERE clause filters
  • Optimize ORDER BY, GROUP BY, and DISTINCT

Without indexes, queries can become slow as the table size grows.

How Indexes Work

When a query uses a column with an index, the database engine can jump directly to matching rows, avoiding a full table scan.

Example: Instead of flipping through every page to find "Chapter 10", you use the book's index to go directly to that page — same concept in SQL.

Syntax to Create an Index

CREATE INDEX index_name
ON table_name (column_name);

Example:

CREATE INDEX idx_lastname
ON employees (last_name);

Now queries like:

SELECT * FROM employees
WHERE last_name = 'Smith';

will be much faster.

Types of Indexes in SQL

Single-Column Index

Indexes a single column.

Composite Index

Indexes multiple columns together.

CREATE INDEX idx_name_dept
ON employees (last_name, department_id);

Unique Index

Ensures all values in the indexed column(s) are unique.

CREATE UNIQUE INDEX idx_email
ON users (email);

Full-Text Index (for text search)

Used for fast searching within large text fields (e.g., in MySQL or PostgreSQL).

When Should You Add an Index?

Add an index if:

  • The column is used frequently in WHERE, JOIN, or ORDER BY
  • The table is large and query speed is critical
  • The column has high cardinality (many unique values)

When Not to Use Indexes

  • On small tables – they won't help and may add overhead
  • On columns with low cardinality (few distinct values, like gender or status)
  • On columns that are frequently updated, as indexes add write overhead

Dropping an Index

To remove an index:

DROP INDEX index_name ON table_name;