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Practice

Subquery in WHERE Clause

A common use of subqueries is inside the WHERE clause.

This allows you to filter results based on the output of another query.


Why It's Useful​

Subqueries inside WHERE help you:

  • Match dynamic values (like the top user, most common course, etc.)
  • Filter without hardcoding
  • Write cleaner logic with fewer joins

Think of it like: “Only show users if they appear in this other result.”


Example: Users who selected more than one course​

The following query returns users who selected more than one course.

Subquery in WHERE clause
SELECT name
FROM users
WHERE user_id IN (
  SELECT user_id
  FROM user_courses
  GROUP BY user_id
  HAVING COUNT(*) > 1
);

Result:​

name
Sofia
Aisha
Liam

These users picked more than one course.


Tips​

  • Always wrap subqueries in ()
  • Use IN or comparison operators (=, >, etc.)
  • Watch out for performance on large datasets

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