SELECT c.name, o.total, p.name

FROM customers c

INNER JOIN orders o ON c.id = o.customer_id

INNER JOIN products p; -- Missing ON clause

SELECT c.name, o.total, p.name

FROM customers c

INNER JOIN orders o ON c.id = o.customer_id

INNER JOIN order_items oi ON o.id = oi.order_id

INNER JOIN products p ON oi.product_id = p.id;

Every join needs its own ON clause. Missing ON clause multiplies result by all rows in that table.

Missing ON clause in multi-table join creates cartesian product

SELECT c.name, o.total, p.name

FROM customers c

LEFT JOIN orders o ON c.id = o.customer_id

INNER JOIN products p ON o.product_id = p.id; -- Wrong join type

SELECT c.name, o.total, p.name

FROM customers c

LEFT JOIN orders o ON c.id = o.customer_id

LEFT JOIN products p ON o.product_id = p.id;

INNER JOIN to products excludes customers without orders, defeating the LEFT JOIN purpose. Use LEFT JOIN consistently.

INNER JOIN after LEFT JOIN eliminates NULL rows from LEFT JOIN

SELECT customers.name, orders.total, products.name

FROM customers

INNER JOIN orders ON customers.id = orders.customer_id

INNER JOIN products ON orders.product_id = products.id;

SELECT c.customer_name, o.total_amount, p.product_name

FROM customers c

INNER JOIN orders o ON c.customer_id = o.customer_id

INNER JOIN products p ON o.product_id = p.product_id;

Use short, meaningful aliases (c, o, p) instead of full table names for better readability.

No table aliases makes multi-table queries verbose and hard to read

SELECT *

FROM customers c

INNER JOIN orders o ON c.id = o.customer_id

INNER JOIN order_items oi ON o.id = oi.order_id

INNER JOIN products p ON oi.product_id = p.id;

SELECT c.customer_name, o.order_date, p.product_name, oi.quantity

FROM customers c

INNER JOIN orders o ON c.id = o.customer_id

INNER JOIN order_items oi ON o.id = oi.order_id