Correlated vs Non-Correlated Subqueries in SQL – What’s the Difference?

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 Subquery?

A subquery is a query nested inside another SQL query. Subqueries can appear in the SELECT, FROM, or WHERE clauses and are used to retrieve data for comparison or filtering.

Subqueries fall into two main categories:

• Non-Correlated Subqueries
• Correlated Subqueries

Understanding the difference is essential for writing efficient and correct SQL queries.

1. Non-Correlated Subquery

Definition:

A non-correlated subquery is independent of the outer query. It runs once and its result is passed to the outer query.

Features:

• Executes only once
• Doesn't rely on data from the outer query
• Can be used in SELECT, WHERE, or FROM clauses

Example:

SELECT name, salary
FROM employees
WHERE salary > (
    SELECT AVG(salary)
    FROM employees
);

This subquery calculates the average salary and compares each employee's salary to it. It does not refer to the outer query, so it's non-correlated.

2. Correlated Subquery

Definition:

A correlated subquery depends on the outer query. It runs once for each row processed by the outer query.

Features:

• References a column from the outer query
• Executes multiple times (once per row)
• Typically used in WHERE or SELECT clauses

Example:

SELECT name, salary
FROM employees e1
WHERE salary > (
    SELECT AVG(salary)
    FROM employees e2
    WHERE e2.department_id = e1.department_id
);

This subquery compares an employee's salary to the average salary within their department, referencing the outer query (e1.department_id). That makes it a correlated subquery.

Key Differences Table

When to Use Which?

Use non-correlated subqueries when you need a fixed result, like a total or average.

Use correlated subqueries when you need row-by-row filtering or calculations based on related data.