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Practice

Subquery in WHERE Clause

One of the most common uses 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 results without hardcoding values
  • Simplify queries without relying on extra joins

You can think of it as: “Return a row only if it exists in another 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 parentheses ()
  • Use IN or comparison operators (=, >, etc.)
  • Watch out for performance on large datasets

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