We show, in practice, the concept of referential integrity, to maintain data consistency. Introduction of foreign keys relative to primary keys and the significance of calling the corresponding attributes in the same manner. Introduction of the difference between the structure of the transaction and the associated physical table (with the concept of “inferred attributes”). Presentation of the selection list as an object automatically created by GeneXus to facilitate work with related data. We show how it is possible to indicate that a foreign key may have no value, through the Nullable property.