Dr. E.F. Codd's 12 Rules

Database

       Database is a collection of data(information) that is organized so that it can easily be accessed, managed, and updated.

Database Management System(DBMS)

       A database management system is a collection of programs written to manage a database. That is, it acts as a interface between user and database.


                                        

Relational Database Management System(RDBMS)

               A Database Management System based on Relational Data Model is known as Relational Database Management System (RDBMS).
               The father of Relational Data Model was Dr. E.F. CODD( a famous mathematician ). He developed the relational data model by taking the concept from Relational Algebra in  June-1970.

               Relational Data Model is nothing but 12 Rules which are named after Codd as Codd Rules.

CODD'S RULES

1. Information Rule: All information in a relational database including table names, column names are represented by values in tables.

2. Guaranteed Access Rule: Every piece of data in a relational database, can be accessed by using combination of a table name, a primary key value that identifies the row and column name which identified a cell.

3.  Systematic Treatment of Nulls Rule: The RDBMS handles records that have unknown or inapplicable values in a pre-defined fashion.

4. Active On-line catalog based on the relational model: The description of a database and in its contents are database tables and therefore can be queried on-line via the data manipulation language. So authorized users must be able to access the database's structure(catalog) using the same query language that they use to access the same database's data

5. Comprehensive Data Sub-language Rule: A RDBMS may support several languages. But at least one of them should allow user to do all of the following: 

                define tables and views,

                query and update the data,

                set integrity constraints,

                set authorizations and 

                define transactions.


6. View Updating Rule: All views that is theoretically updateable can be update using RDBMS.Data consistency is ensured since the changes made in the view are transmitted to the base table and vice-versa.

7. High-level Insert, Update and Delete: The RDBMS  supports insertions, updation and deletion at a table level. The performance is improved since the commands act on a set of records rather than one record at a time.

8. Physical Data Independence: The execution of adhoc requests and application programs is not affected by changes in the physical data access and storage methods.Database  administrators can make changes to the physical access and storage method which improve performance and do not require changes in the application programs or requests. Here the user specified what he wants an need not worry about how the data is obtained.


9. Logical Data Independence: Logical changes in tables and views such adding/deleting columns or changing fields lengths need not necessitate modifications in the programs or in the format of adhoc requests.
      For example, adding attribute or column to the base table should not disrupt the programs or the interactive command that have no use for the new attribute.


10. Integrity Independence: Like table/view definition, integrity constraints are stored in the on-line catalog and can therefore be changed without necessitating changes in the application programs.


11. Distribution Independence: Application programs and adhoc requests are not affected by change in the distribution of physical data. Improved systems  reliability since application programs will work even if the programs and data are moved in different sites.


12. No subversion Rule: If the RDBMS has a language that accesses the information of a record at a time, this language should not be used to bypass the integrity constraints. This is necessary for data integrity.