would allow insertion of rows where author_last or author_first are NULL irrespective of how the table Authors is defined or what it contains. More precisely, a null in any of these fields would allow any value in the other one, even on that is not found in Authors table. For example, if Authors contained only ('Doe', 'John'), then ('Smith', NULL) would satisfy the foreign key constraint. SQL-92 added two extra options for narrowing down the matches in such cases. If codice_42 is added after the codice_43 declaration then any non-null must match the foreign key, e. g. ('Doe', NULL) would still match, but ('Smith', NULL) would not. Finally, if codice_44 is added then ('Smith', NULL) would not match the constraint either, but (NULL, NULL) would still match it.