UNIQUE Constraint in SQL – Ensuring Distinct Data Entries

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 the UNIQUE Constraint?

The UNIQUE constraint in SQL ensures that all values in a column (or a combination of columns) are distinct. It prevents duplicate values from being inserted into the specified column(s), maintaining data accuracy and integrity.

Why Use UNIQUE?

Use UNIQUE when:

• A column should contain only distinct values
• You need to enforce business rules (e.g., one email per user)
• You want to prevent duplicates in non-primary key columns

UNIQUE Constraint Syntax

1. Add UNIQUE During Table Creation

CREATE TABLE users (
  id INT PRIMARY KEY,
  email VARCHAR(100) UNIQUE,
  username VARCHAR(50) UNIQUE
);

Both email and username must be unique in the users table.

2. Add UNIQUE to Existing Table

ALTER TABLE users
ADD CONSTRAINT unique_email UNIQUE (email);

You can also name the constraint for easier management.

UNIQUE vs PRIMARY KEY

So while every primary key is unique, not every unique column is a primary key.

Example: UNIQUE with INSERT

INSERT INTO users (id, email)
VALUES (1, 'test@example.com');

-- This will fail if the same email already exists
INSERT INTO users (id, email)
VALUES (2, 'test@example.com');

The second insert will throw an error due to the UNIQUE constraint.

Composite UNIQUE Constraints

You can enforce uniqueness across multiple columns together:

CREATE TABLE enrollments (
  student_id INT,
  course_id INT,
  UNIQUE(student_id, course_id)
);

This ensures a student can't be enrolled in the same course more than once.

Dropping a UNIQUE Constraint

ALTER TABLE users
DROP INDEX unique_email;

Note: The exact syntax may vary slightly depending on the database system.