What is a Subquery?

A subquery is a SQL query written inside another query.

It allows you to use the result of one query as input for another, making your SQL logic more flexible and powerful.

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 creating 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'
);

Here, the subquery in the WHERE clause filters the users table so that only users who selected the Python course are returned.