How do INNER JOIN and LEFT JOIN differ when a customer has no orders?

An INNER JOIN retu s customer-order combinations only when the join condition matches. A LEFT JOIN retains every customer from the left side and uses NULL for order columns when no match exists. Placing a right-table filter in WHERE can remove those NULL-extended rows and unintentionally change the result. Put a condition in ON when it should restrict matched orders while preserving customers. Also consider multiple orders per customer: a join can legitimately retu several rows, so choose aggregation or existence checks according to the required output.