Module 5: Advanced Query Logic

As your data questions become more complex, your queries will naturally become longer and harder to read. In this module, you will learn how to move away from “Spaghetti SQL” by using subqueries and Common Table Expressions (CTEs) to break complex logic into modular, readable, and reusable blocks.

Learning Objectives

In this module, you will:

• Master the different types of subqueries: Scalar, Table, and Correlated.
• Understand the performance implications of EXISTS vs. IN.
• Learn how to use Common Table Expressions (CTEs) to write cleaner, “top-down” code.
• Query hierarchical and tree-structured data using Recursive CTEs.

Chapters

5.1 Subqueries Nesting queries inside other queries.

5.2 Common Table Expressions Using the WITH clause for readable, modular SQL.

5.3 Recursive CTEs Traversing hierarchical data like org charts and trees.

Note

CTEs and Subqueries often achieve the same result, but CTEs are almost always preferred in modern SQL for their readability and maintainability.