Subqueries in SELECT, FROM, and WHERE Clauses – SQL 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

What Are Subqueries?

A subquery is a query nested inside another query. In SQL, subqueries can appear in various parts of a query:

• In the SELECT clause (returning scalar values)
• In the FROM clause (acting as a temporary table)
• In the WHERE clause (used for filtering)

Let's break down each one with examples.

1. Subquery in the SELECT Clause

Use this when you need to add a calculated value to each row in your results.

Example:

SELECT 
  name,
  salary,
  (SELECT AVG(salary) FROM employees) AS avg_salary
FROM employees;

This adds a new column showing the average salary for reference, using a scalar subquery.

2. Subquery in the FROM Clause

Subqueries in the FROM clause act as derived tables or inline views.

Example:

SELECT department_id, total_salary
FROM (
  SELECT department_id, SUM(salary) AS total_salary
  FROM employees
  GROUP BY department_id
) AS dept_totals
WHERE total_salary > 100000;

The subquery groups salaries by department, and the outer query filters based on those totals.

3. Subquery in the WHERE Clause

This is the most common use case — filtering rows based on the result of another query.

Example:

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

This returns only those employees who earn above the average salary.

Combined Use: SELECT + FROM + WHERE

You can combine subqueries across clauses for more complex logic:

SELECT name,
       (SELECT department_name 
        FROM departments d 
        WHERE d.id = e.department_id) AS dept_name
FROM (
  SELECT * 
  FROM employees
  WHERE salary > 50000
) AS e;

This filters employees with salary > 50,000 then adds their department name using a subquery in the SELECT clause

Best Practices for Using Subqueries

• Always alias subqueries in the FROM clause
• Ensure subqueries in SELECT and WHERE return a single value (scalar)
• Use joins or CTEs when subqueries become too complex or affect performance
• Test subqueries individually to isolate issues