Scalar Subqueries in SQL – Return a Single Value

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 Scalar Subquery?

A scalar subquery is a type of subquery that returns exactly one value — a single row and single column.

It is often used in:

• The SELECT clause
• The WHERE clause
• The HAVING clause

Scalar subqueries are ideal when you need a calculated or derived value for each row in your result set.

Basic Syntax

SELECT column1,
       (SELECT expression FROM table WHERE condition) AS scalar_value
FROM main_table;

Example Table: employees

Example 1: Compare Each Salary to the Company Average

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

Result:

The scalar subquery (SELECT AVG(salary) FROM employees) runs once and returns a single value, which is used for each row.

Example 2: Using Scalar Subquery in WHERE Clause

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

Result:

This filters out rows where the salary is greater than the average salary.

Rules for Scalar Subqueries

• Must return only one row and one column
• Will cause an error if it returns more than one value
• Can be nested within SELECT, WHERE, or HAVING clauses
• Often used in performance comparisons, scoring, or dynamic filtering

Use Cases

• Compare a value to a global or conditional average/maximum
• Fetch a related summary for each row
• Add computed fields to result sets dynamically

Best Practices

• Ensure your subquery returns only one row and column
• Use LIMIT 1 if necessary to enforce single-row results
• Test subqueries independently to avoid logic errors