Subqueries and EXISTS

Subqueries and EXISTS

Now that we've covered filtering in Filtering: LIKE, IN, BETWEEN, AND, OR, NOT, combining tables in Joins in SQL, and grouped summaries in Grouping in SQL, we're ready for a pattern that appears constantly in SQL interviews: asking one query to help another query make a decision.

That helper query is called a subquery.

A subquery can answer questions like:

• Which customers placed at least one order?
• Which products are more expensive than the average product?
• Which users have no activity records?
• Which departments have employees earning above the company average?

The key idea is simple: a subquery is a query nested inside another SQL statement. The outer query uses the inner query's result to filter, compare, or check for existence.

In this lesson, we will focus on subqueries in WHERE and the interview-critical EXISTS pattern.