Module 3: Aggregation and Grouping

So far, we have worked with individual rows. However, business decisions are rarely made by looking at a single transaction. Instead, we need to summarize data: What are the total sales? What is the average order value? Which department has the most employees?

In this module, you will learn how to “collapse” multiple rows into summary statistics.

Learning Objectives

In this module, you will:

• Master the five core aggregate functions: COUNT, SUM, AVG, MIN, and MAX.
• Understand the “Rule of Aggregation” when using the GROUP BY clause.
• Learn the critical difference between WHERE and HAVING.
• Handle duplicates effectively using DISTINCT.

Chapters

3.1 Aggregate Functions Reducing many rows into a single value.

3.2 The GROUP BY Clause Summarizing data by categories or dimensions.

3.3 Filtering Aggregates The critical distinction between WHERE and HAVING.

3.4 Distinct Values Managing duplicates and unique counts.

Caution

Aggregation is a “lossy” process. When you aggregate, you lose access to the individual row details unless you use advanced techniques like Window Functions (which we cover in Module 6).