Using Subqueries in JOINs – SQL Explained with 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

Why Use Subqueries in JOINs?

Sometimes, you may want to join a table with a summarized or filtered result. Instead of joining directly with a full table, you can use a subquery that:

Aggregates data (e.g., totals, averages)

Filters rows before the join

Creates a derived temporary table

This helps simplify logic and improve performance in complex queries.

Syntax – JOINing with a Subquery

SELECT a.column, b.column
FROM table_a AS a
JOIN (
    SELECT column, AGG_FUNC(value) AS result
    FROM table_b
    GROUP BY column
) AS b ON a.column = b.column;

Example Scenario

Let's say you have two tables:

customers

id name

1 Alice

2 Bob

3 Charlie

orders

id customer_id amount

1 1 200

2 1 300

3 2 150

4 3 100

Example: Join with Aggregated Subquery

Join customers with total order amount per customer:

SELECT c.name, o.total_spent
FROM customers c
JOIN (
    SELECT customer_id, SUM(amount) AS total_spent
    FROM orders
    GROUP BY customer_id
) AS o ON c.id = o.customer_id;

Result:

name total_spent

Alice 500

Bob 150

Charlie 100

The subquery calculates totals, and the outer query joins that with customer names.

When to Use Subqueries in JOINs

When you need to aggregate data before joining

When you want to filter or transform data before combining tables

When building reports or dashboards that require grouped values

Tips for Using Subqueries in JOINs

Always alias your subqueries

Subqueries in JOIN should return only necessary columns

Make sure the join keys match correctly between outer and inner queries

Use indexes on join columns for performance

Alternative: Common Table Expressions (CTEs)

CTEs can often improve readability when using subqueries in JOINs:

WITH order_totals AS (
    SELECT customer_id, SUM(amount) AS total_spent
    FROM orders
    GROUP BY customer_id
)
SELECT c.name, o.total_spent
FROM customers c
JOIN order_totals o ON c.id = o.customer_id;

Same logic, but more modular and easier to maintain.