CHECK Constraint in SQL – Validating Column Values

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 CHECK Constraint?

The CHECK constraint in SQL is used to restrict the values that can be inserted into a column. It ensures that data meets specific conditions before it's added to the table. If the data violates the rule, SQL throws an error and blocks the insert or update.

Why Use CHECK Constraints?

• To enforce business rules at the database level
• To prevent invalid or out-of-range data
• To avoid the need for repetitive application-side validation
• To maintain data consistency and integrity

Syntax of CHECK Constraint

1. Add CHECK During Table Creation

CREATE TABLE employees (
  id INT PRIMARY KEY,
  name VARCHAR(100),
  age INT CHECK (age >= 18),
  salary DECIMAL(10, 2) CHECK (salary >= 0)
);

The age column must be 18 or older

The salary must be non-negative

2. Named CHECK Constraint

CREATE TABLE products (
  id INT PRIMARY KEY,
  price DECIMAL(10, 2),
  CHECK (price > 0)
);

Or with a custom name:

CONSTRAINT chk_price CHECK (price > 0)

3. Add CHECK to Existing Table

ALTER TABLE employees
ADD CONSTRAINT chk_age CHECK (age >= 18);

4. Drop a CHECK Constraint

ALTER TABLE employees
DROP CONSTRAINT chk_age;

Note: Constraint naming and syntax can vary by database (e.g., MySQL, PostgreSQL, SQL Server)

Examples of Common CHECK Constraints

CHECK vs Triggers or Application Logic

Use CHECK when you can define rules with simple expressions — it's more efficient and portable.