SQL Subqueries

Introduction

This lesson covers Subqueries in SQL. You need it for almost every real program you will write.

Subqueries is a core topic on the SQL learning path. Work through the examples on your own machine — that is where things actually click.

Syntax

Basic Example

Code Explanation

1. Read the code top to bottom
2. Identify inputs and outputs
3. Run it locally and change one line

Real-World Usage

Subqueries appears in everyday SQL projects — apps, scripts, sites, and tools.

• Production codebases
• Open-source libraries
• Interview questions
• Your own side projects

Common Mistakes

Copy-pasting without typing. Your fingers need the repetition.

Ignoring error messages. They usually point to the exact line.

Best Practices

• Type every example yourself instead of only reading
• Change values in the sample code and see what breaks
• Fix errors yourself before peeking at solutions

Tips

Tip: Keep a notes file with syntax snippets you use often for Subqueries.

Summary

1. You studied Subqueries in SQL.
2. Practice with the exercises at the bottom of this page.
3. Move to the next lesson when you can write the examples from memory.