Codd's Rule for Relational DBMS

Relational data model is the primary data model, which is used widely around the world for data storage and processing. This model is simple and it has all the properties and capabilities required to process data with storage efficiency. Relational Model was proposed by E.F. Codd to model data in the form of relations or tables.

Codd's Rule for Relational DBMS:

1. Information Rule: Data stored in Relational model must be a value of some cell of a table. Everything in a database must be stored in a table format.

2. Guaranteed Access Rule: Every data element (value) must be accessible by combination of table name, primary-key (row value), and attribute-name (column value).

3. Systematic Treatment of NULL values: The NULL values in a database must be given a systematic and uniform treatment. NULL value in database must only correspond to missing, unknown or not applicable values.

4. Active Online Catalogue: Structure of database must be stored in an online catalogue, known as data dictionary, which can be accessed by authorized users.

5. Comprehensive Data Sub-language Rule: A database should be accessible by a language supported for definition, manipulation and transaction management operation. If the database allows access to data without any help of this language, then it is considered as a violation.

6. View Updating Rule: Different views created for various purposes should be automatically updatable by the system.

7. High level insert, update and delete rule: Relational Model should support insert, delete, update etc. operations at each level of relations. Also, set operations like Union, Intersection and minus should be supported.

8. Physical data independence: Any modification in the physical location of a table should not enforce modification at application level. The data stored in a database must be independent of the applications that access the database.

9. Logical data independence: Any modification in logical or conceptual schema of a table should not enforce modification at application level. For example, merging of two tables into one should not affect application accessing it which is difficult to achieve.

10. Integrity Independence: Integrity constraints modified at database level should not enforce modification at application level. This rule makes a database independent of the front-end application and its interface.

11. Distribution Independence: Distribution of data over various locations should not be visible to end-users. This rule has been regarded as the foundation of distributed database systems.

12. Non-Subversion Rule: Low level access to data should not be able to bypass integrity rule to change data.