202 — Joining Tables

Intermediate

Connect related tables to build complete pictures of your data. Joins are the most powerful tool in SQL — they let you answer questions that span multiple tables, like 'what did each customer order and how much did they spend?'

Learning Objectives

Understand why relational databases split data across tables

Write INNER JOIN queries with proper ON conditions

Use LEFT JOIN to include rows with no matches

Join three or more tables in a single query

Combine JOINs with GROUP BY for aggregated reports

Recognize and avoid accidental Cartesian products

Notice when one-to-many joins multiply rows before aggregation

Step 1

Why joins exist

Examine how the e-commerce database splits data across tables and why you need joins to put it back together.

Commands to Run

SELECT * FROM orders LIMIT 5;

SELECT * FROM customers LIMIT 5;

What This Does

Notice that the orders table stores customer_id, not the customer's name or email. This is called normalization — storing each fact in one place to avoid duplication and inconsistency. If a customer changes their email, you update one row in customers rather than thousands of rows in orders. The trade-off is that you need JOINs to reassemble the full picture.

Expected Outcome

The orders table shows customer_id as a number, not a name. To see who placed each order, you need to look up that ID in the customers table.

Pro Tips

1
Foreign keys (like customer_id in orders) are the 'links' between tables
2
Think of each table as a spreadsheet tab — JOINs let you combine tabs based on matching values

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