What is a Subquery?

A subquery is a SQL query written inside another query.

It lets you use one query's result as input for another — making your SQL logic smarter and more dynamic.

Subqueries are also called nested queries or inner queries.

You can use subqueries when you want to:

• Filter or compare based on dynamic results
• Avoid repeating steps or temporary tables
• Simplify logic inside WHERE, FROM, or SELECT clauses

Tip: Always wrap subqueries in parentheses.

In this example, we work with two tables:

• users: contains user IDs and names
• user_courses: tracks which courses each user selected

To find users who selected the Python course, we can use a subquery like this:

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SELECT name
FROM users
WHERE user_id IN (
  SELECT user_id
  FROM user_courses
  WHERE course = 'Python'
);

The subquery in the WHERE clause filters the users table to only include users who have selected the Python course.