Codd’s Relational Rules - Database Design and Development: An Essential Guide for IT Professionals - page 680

Appendix B

Codd's Relational Rules

In 1985, Dr. E. F. Codd, the acknowledged “father” of the relational data model,
presented the following 12 rules that a database must obey if it is to be considered
truly relational.

Rule 1: The information rule All information in a relational database is repre-
sented explicitly at the logical level and in exactly one way—by values in tables.

Rule 2: Guaranteed access rule Each and every datum (atomic value) in a rela-
tional database is guaranteed to be logically accessible by resorting to a combina-
tion of table name, primary key value, and column name.

Rule 3: Systematic treatment of null values Null values (distinct from an empty
character string or a string of blank characters and distinct from zero or any other
number) are supported for representing missing information and inapplicable infor-
mation in a systematic way, independent of the data type.

Rule 4: Dynamic online catalog based on the relational model The database
description is represented at the logical level in the same way as ordinary data, so
that authorized users can apply the same relational language to its interrogation as
they apply to the regular data.

Rule 5: Comprehensive data sublanguage rule A relational system may support
several languages and various modes of terminal use (for example, the fill-in-the-