Up to this point, when two tables were joined, one row in the first table matched exactly one row in the second table. making the joins one-to-one joins. Imagine if there was more than one salesorder row for a customer id. Multiple order numbers would be printed. That would be a one-to-many join, where one customer row joins to more than one salesorder row. Suppose there were no orders made by a customer. Even though there was a valid customer row, if there was no salesorder row for that customer id number, no rows would be returned. We could call that a one-to-none join.8.5
Figure shows an example.