Introduction to JOINs

Real databases have multiple related tables. JOINs let you combine them to get complete information.

New Tables Available

Starting this module, you have access to order-related tables:

• orders - Customer orders (user_id, order_date, total_amount, status)
• order_items - Items in each order (order_id, product_id, quantity, price)

These tables reference each other through ID columns (foreign keys).

Why JOIN?

Consider this: the orders table has user_id but not the user's name. The products table has product names but order_items only has product_id. JOINs let you combine these to answer questions like "What did Alice order?"

Basic JOIN Syntax

SELECT columns
FROM table1
JOIN table2 ON table1.column = table2.column;

The ON clause specifies how the tables relate.

Exercises

Table Aliases

Use short aliases to make JOINs more readable:

SELECT o.id, u.name
FROM orders o
JOIN users u ON o.user_id = u.id;

Free Practice

Try:

• Join products with categories to see category names
• Join order_items with orders to see order totals
• Experiment with different column selections