201 — Aggregation and Grouping

Intermediate

Learn to summarize data with aggregate functions and GROUP BY. Transform thousands of rows into meaningful totals, averages, and counts — the foundation of every business report.

Learning Objectives

Use COUNT, SUM, AVG, MIN, and MAX to summarize data

Group rows with GROUP BY to create per-category summaries

Understand the difference between WHERE and HAVING

Combine grouping with sorting for polished reports

Count distinct values within groups

Build real-world reports using aggregation

Recognize the grain of a grouped result: one row per group

Step 1

Count rows with COUNT(*)

Start with the simplest aggregate: counting how many rows exist in a table.

Commands to Run

SELECT COUNT(*) AS total_customers FROM customers;

SELECT COUNT(*) AS total_orders FROM orders;

SELECT COUNT(*) AS total_products FROM products;

What This Does

COUNT(*) counts every row in the result set, including rows with NULL values. The AS keyword gives the output column a readable name (an alias). Aggregate functions collapse many rows into a single summary row.

Expected Outcome

You should see one row per query with the total count. For example, total_customers might show 50, total_orders might show 200, depending on your seed data.

Pro Tips

1
Always alias your aggregates with AS — 'COUNT(*)' as a column header is not helpful to anyone reading your report
2
COUNT(*) is the fastest way to count rows because the database doesn't need to inspect individual column values
3
Result grain here is simple: one aggregate query with no GROUP BY returns one summary row

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