Nested Query

When a customer service analyst wants to find all customers who ordered the most expensive product in the store, a basic query won’t cut it. We need to first find the maximum price, and then check who bought it—a task that requires one query to be embedded within another. This is where Nested queries, also known as subqueries, become essential. Nested queries help us solve problems where one result depends on another, and they appear in everything from fraud detection to leaderboards to filtering by computed metrics.

In this lesson, we will:

• Explore the power and flexibility of nested queries.

• Be able to filter records based on dynamic criteria, calculate values based on inner queries, and understand how subqueries make our code more readable and efficient.

• Work through practical examples, study how they’re structured, and get plenty of hands-on practice.

Pattern overview

Category:

• Comparison Patterns

Intent:

Use a query inside another query to perform conditional logic, filtering, or calculations based on dynamic results.

Motivation:

Sometimes we don’t know the value we need to filter or compare against ahead of time. Nested queries allow us to compute this value on the fly and use it directly in our outer query. This is key for writing flexible, reusable, and intelligent SQL.

Also known as:

• Subqueries

• Inner Queries

• Correlated Subqueries (for row-wise comparisons)

Structure

There are two common forms of nested queries:

1. Scalar subquery: It returns a single value, usually for filtering or comparison.