You can define a relationship between two tables if any of the fields in one of the secondary table’s keys are also present in the primary table. The fields the relationship can be based on don’t need to be part of a key in the primary table, nor do they have to use the same global field. However, they must be based on compatible data types.
For example, in a Real Estate tracking application, a relationship could be defined between the Sales table and the Customer List table, based upon the Buyer Name field in the Sales table and the Customer Name field in the Customer List table.