Codd's Rule

                                              Codd’s Rule for RDBMS

0. (Yes, there is a Rule Zero)

For a system to qualify as a RDBMS, that system must 

use its RELATIONAL facilities (exclusively) to MANAGE the DATABASE.

1 Information rule:  

               All information in a relational database is represented explicitly at the logical 

Level and in exactly one way, that is by values in tables.

               In Short: (Informal definition of a relational database.)

2 Guaranteed access rule: 

               Each and every value in a relational database is guaranteed to be logically accessible

 by resorting to a combination of table name, column name primary and unique key value, 

               (Importance of primary keys for locating data in the 

Database and the primary key value find the row containing an individual data item of interest)

3 Treatment of NULL values. 

          NULL values (Distinct from an empty character string 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.

                               (Requires support for missing data through NULL values.)

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.

               (Requires that database be self-describing. In other words, the database must contain 

certain system tables whose columns describe the structure of the database itself.)

5 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). 

               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 & View  definition

                               • Data manipulation (interactive and by program)

                               • Transaction boundaries (begin, commit, and rollback)

               (Mandates using a relational database language, such as SQL. 

DBMS support: support all the central functions of a DBMS -creating a database, 

retrieving and entering data, implementing database security)

6 View updating rule. 

          All views that are theoretically update able are also update able by the system.

               (Deals with views, which are virtual tables used to give various 

users of a database different views of its structure. “No update via Views”)

7 High-level Insert, Update and delete. 

               The capability of handling a base relation or a derived relation as a single              

Operand applies to the insertion, update, and deletion of data.

               (The system must support set-at-a-time INSERT, UPDATE, and DELETE operators.)

8 Physical Data Independence. 

Application programs and terminal activities remain logically unimpaired 

whenever any changes are made in either storage representations or access 

methods.

                               (Storage techniques used by the DBMS, and changes to the 

structure of the tables in the database, should not affect the user's

Ability to work with the data.)

9. Logical data independence.

               Application programs and terminal activities remain logically unimpaired 

when information preserving changes of any kind that theoretically permits 

Unimpairment is made to the base tables.

               (Database must be completely independent of the application)

11 Distribution independence. 

               The database language must be able to manipulate distributed 

data location on other computer systems.

               (A relational DBMS has distribution independence.)

12 Nonsubversion rule. 

               If a relational system has a low-level (single record at a time) language,

 that cannot expressedin the higher level relational language (multiple records at a time).