Relational Database Model

E.F. Codd's 12 Rules

1985, E.F. Codd published 12 Rules about "True Relation Database" that accepted and implemented most of the rules in different Relational Database Management System.

01. The Information Rule

All information in a relational database (including Table and Column Names) is represented explicitly at the logical level and in exactly one way - by values in tables.

02. Guaranteed Access Rule

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

03. 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 in a fully relational DBMS for representing missing information and inapplicable information in a systematic way, independent of the data type.

04. 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.

05. 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:

     01. Data Definition
     02. View Definition
     03. Data Manipulation
     04. Integrity Constraints
     05. Authorization
     06 Transaction Boundaries

06. View Updating Rule

All views that are theoretically updatable must be updatable by the system.

07. 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.

08. Physical Data Independence

Application programs and terminal activities remain logically unimpaired whenever any changes are made in either storage representations or access methods.

09. 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.

10. Integrity Independence Rule

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.

11. Distribution Independence Rule

The distribution of portions of the database to various locations should be invisible to users of the database. Existing applications should continue to operate successfully.

12. 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).

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