Codd’s 12 rules is a set of rules that a database management system (DBMS) must satisfy if it’s to be considered relational (i.e. a relational DBMS).

The rules were proposed by Edgar F. Codd, who is considered a pioneer of the relational database model.

Codd’s 12 rules is actually a set of thirteen rules, numbered from zero to twelve. The twelve rules are based on a single foundation rule — Rule Zero.

Codd’s 12 rules are as follows.

Rule 0: The foundation rule:

For any system that is advertised as, or claimed to be, a relational data base management system, that system must be able to manage data bases entirely through its relational capabilities.

Rule 1: The information rule:

All information in a relational data base is represented explicitly at the logical level and in exactly one way — by values in tables.

Rule 2: The guaranteed access rule:

Each and every datum (atomic value) in a relational data base is guaranteed to be logically accessible by resorting to a combination of table name, primary key value and column name.

Rule 3: Systematic treatment of null values:

Null values (distinct from the empty character string or a string of blank characters and distinct from zero or any other number) are supported in fully relational DBMS for representing missing information and inapplicable information in a systematic way, independent of data type.

Rule 4: Dynamic online catalog based on the relational model:

The data base 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: The comprehensive data sublanguage rule:

A relational system may support several languages and various modes of terminal use (for example, the fill-in-the-blanks mode). However, there must be at least one language whose statements are expressible, per some well-defined syntax, as character strings and that is comprehensive in supporting all of the following items:

• Data definition.
• View definition.
• Data manipulation (interactive and by program).
• Integrity constraints.
• Authorization.
• Transaction boundaries (begin, commit and rollback).

Rule 6: The view updating rule:

All views that are theoretically updatable are also updatable 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 not 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 representations 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 data base must be definable in the relational data sublanguage and storable in the catalog, not in the application programs.

Rule 11: Distribution independence:

The end-user must not be able to see that the data is distributed over various locations. Users should always get the impression that the data is located at one site only.

Rule 12: The nonsubversion rule:

If a relational system has a low-level (single-record-at-a-time) language, that low level cannot be used to subvert or bypass the integrity rules and constraints expressed in the higher level relational language (multiple-records-at-a-time).

What’s the Purpose of Codd’s 12 Rules?

Not all database management systems (DBMS) are designed to be relational. For example, there are flat file databases, hierarchical databases, object oriented databases, and a whole bunch of different NoSQL databases.

But if a DBMS is designed as a relational database, it generally has certain characteristics. For example, relational databases use tables to store data, and these tables can have a relationship between them. But there are many other factors that distinguish relational databases from non-relational.

Codd’s 12 rules were established to specify a set of criteria that could be used to determine if a DBMS could be considered relational or not. If a DBMS complied with all of Codd’s rules, then it could be considered a relational DBMS (i.e. RDBMS). Otherwise it couldn’t.

The rules were written in an attempt to preserve the original vision of the relational model. Codd invented the relational model in 1970, but by the time the rules were written in the mid-1980s, there had been a tendency among database vendors to repackage their existing DBMS products as RDBMS without necessarily making the product truly relational as defined by Codd.

Even today, few “relational” database management systems comply with all of Codd’s rules, despite being marketed as relational. Because of this, some purists refer to these systems as pseudo-relational database management systems (PRDBMS), while referring to any system that satisfies all Codd’s 12 rules as truly-relational database management systems (TRDBMS).

In the early 1990s, C.J. Date and Hugh Darwen began writing The Third Manifesto in an attempt to provide a blueprint for the design of future DBMSs, while reinforcing the relational model proposed by Codd.