SQL Multi-Table JOINs Practice Exercises

What is a JOIN?

A JOIN combines rows from two or more tables based on a related column between them — usually an ID that appears in both. Real-world data is normally split across separate tables like this, so joins are how you bring related information back together in one result.

-- Combine order data with the customer who placed each order
SELECT o.order_id, c.customer_name, c.city, o.book_title, o.price_usd
FROM bookstore_orders o
JOIN bookstore_customers c ON o.customer_id = c.customer_id;

The ON clause tells SQL how the two tables relate — here, bookstore_orders.customer_id matches bookstore_customers.customer_id. Plain JOIN (also written INNER JOIN) only returns rows where a match exists in both tables.

LEFT JOIN — keeping unmatched rows

LEFT JOIN keeps every row from the left (first) table, whether or not it has a match in the right table. When there's no match, the right table's columns come back as NULL instead of the row being dropped.

-- Every customer, including those who have never placed an order
SELECT c.customer_name, o.book_title, o.order_date
FROM bookstore_customers c
LEFT JOIN bookstore_orders o ON c.customer_id = o.customer_id;

Any customer with no matching row in bookstore_orders still appears once, with book_title and order_date shown as NULL.

Joining three or more tables

You can chain as many JOIN clauses as needed. Each one links a new table using its own ON condition, and the result flows through — a table joined later can still be connected back to any table joined earlier.

-- Combine rental, customer, and car details in one result
SELECT r.rental_id, cu.customer_name, ca.car_model, r.rental_date
FROM car_rentals r
JOIN rental_customers cu ON r.customer_id = cu.customer_id
JOIN rental_cars ca ON r.car_id = ca.car_id;

Here, car_rentals is the hub table connecting to both rental_customers and rental_cars — each joined on its own foreign key.

Common mistakes

Forgetting the ON clause — a JOIN without a matching condition produces a cartesian product, pairing every row in one table with every row in the other. Always pair a JOIN with an ON.

Ambiguous column names — when two joined tables share a column name (like customer_id), you must prefix it with a table alias, or SQL won't know which one you mean.

Using INNER JOIN when you need LEFT JOIN — an INNER JOIN silently drops rows with no match. If you need to see every customer even without orders, that's a LEFT JOIN, not an INNER JOIN.

Losing track of which table a column belongs to — once you're joining three or more tables, always alias each table and prefix every column, even ones that aren't ambiguous yet — it keeps the query readable as it grows.