Module 4: Multi-Table Systems

Real-world databases are normalized, meaning data is split into multiple specialized tables to reduce redundancy. To answer any meaningful business question, you must learn how to stitch these tables back together.

This module covers the most challenging transition for beginners: moving from querying a single table to managing complex multi-table relationships.

Learning Objectives

In this module, you will:

• Understand the mathematical foundations of Set Theory.
• Master all major JOIN types: INNER, LEFT, RIGHT, FULL, and CROSS.
• Learn how to query hierarchical data using Self-Joins.
• Use Set Operators (UNION, INTERSECT, EXCEPT) to combine result sets.

Chapters

4.1 Set Theory Basics The logic of Union, Intersection, and Difference.

4.2 JOIN Operations Stitching tables together: INNER, LEFT, and more.

4.3 Self-Joins Joining a table to itself for hierarchical analysis.

4.4 Set Operators Combining query results with UNION and EXCEPT.

Tip

Joins are where SQL’s performance most frequently degrades. Understanding the mechanics of how tables “find” each other is key to writing scalable queries.