According to Elmasri and Navathe (1994), Dr. E. F. Codd, the originator of the relational data model, published a two-part article in ComputerWorld (Codd, 1985) that lists 12 rules for how to determine whether a DBMS is relational and to what extent it is relational. These rules provide a very useful yardstick for evaluating a relational system. Codd also mentions that, according to these rules, no fully relational system is available yet. In particular, rules 6, 9, 10, 11, and 12 are difficult to satisfy.
THE 12 RULES
Rule 1: The Information Rule
All information in a relational database is represented explicitly at the logical level in exactly one way—by values in tables.
Rule 2: Guaranteed Access Rule
Each and every datum (atomic value) in a relational database is guaranteed to be logically accessible by resorting to a table name, primary key value, and column name.
Rule 3: Systematic Treatment of Null Values
Null values (distinct from empty character string or a string of blank characters and distinct from zero or any other number) are supported in the fully relational DBMS for representing missing information in a systematic way, independent of data type.
Rule 4: Dynamic On-line Catalog Based on the Relational Model
The database description is represented at the logical level in the same way as ordinary data, so authorized users can apply the same relational language to its interrogation as they apply to 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-blanks mode). However, there must be at least one language whose statements are expressible, per some well-defined syntax, as character strings and whose ability to support all of the following is comprehensible: data definition, view definition, data manipulation (interactive and by program), integrity constraints, and transaction boundaries (begin, commit, and rollback).
Rule 6: View Updating Rule
All views that are theoretically updateable are also updateable by the system.
Rule 7: High-level Insert, Update, and Delete
The capability of handling a base relation or a derived relation as a single operand applies nor only to the retrieval of data but also to the insertion, update, and deletion of data.
Rule 8: Physical Data Independence
Application programs and terminal activities remain logically unimpaired whenever any changes are made in either storage representation or access methods.
Rule 9: Logical Data Independence
Application programs and terminal activities remain logically unimpaired when information preserving changes of any kind that theoretically permit unimpairment are made to the base tables.
Rule 10: Integrity Independence
Integrity constraints specific to a particular relational database must be definable in the relational data sublanguage and storable in the catalog, not in the application programs.
A minimum of the following two integrity constraints must be supported:
2. Referential integrity: For each distinct nonnull foreign key value in a relational database, there must exist a matching primary key value from the same domain.
Rule 11: Distribution Independence
A relational DBMS has distribution independence. Distribution independence implies that users should not have to be aware of whether a database is distributed.
Rule 12: Nonsubversion Rule
If a relational system has a low-level (single-record-at-a-time) language, that low-level language cannot be used to subvert or bypass the integrity rules or constraints expressed in the higher-level (multiple-records-at-a-time) relational language.
Note: There is a rider to these 12 rules known as Rule Zero: "For any system that is claimed to be a relational database management system, that system must be able to manage data entirely through its relational capabilities."
On the basis of the above rules, there is no fully relational DBMS available today.
Codd, E. (1985). "Is Your DBMS Really Relational?" and "Does Your DBMS Run By the Rules?" ComputerWorld, October 14 and October 21.
Elmasri, R., & Navathe, S. (1994). Fundamentals of Database Systems. 2nd ed. Redwood City, CA: The Benjamin/Cummings Publishing Co. pp. 283 – 285.