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Friday, April 29, 2011
What is codd’s rule ?
Dr. E.F. Codd, an IBM researcher, first developed the relational data model in 1970. Over time it has proved to be flexible, extensible, and robust. In 1985 Codd published a list of 12 rules known as "Codd's 12 Rules" that defined how a true RDBMS should be evaluated. Understanding these rules will greatly improve the ability to understand RDBMS's in general, including Oracle. Also note that these rules are "guidelines" because, to date, no commercial relational database system fully conforms to all 12 rules .
It is important to remember that RDBMS is not a product, it is a method of design. Database products like the Oracle Database, SQL Server, DB2, Microsoft Access, etc. all adhere to the relational model of database design. The rules are as follows :
1. ) The Information Rule: For a database to be relational all information is represented as data values .
2.)The Rule of Guaranteed Access: The data represented in table by using table name, column name and primary key value defined for that table.
3.)The systematic treatment of null values:If a information is not present then they are represented as null values in database. But it is vital to note that primary key values cannot be not null and also nul values are different from spaces or zeroes.
4.)The database Description Rule: The description f database is also maintained in a place called as data dictionary and users can access this if they have proper authority or privilege to do the same.
5.)The comprehensive data sublanguage rule : Database must support the following namelyData definition .
6.)The view updating rule : All views that are updatable by theory can also be updated by the system.
7.) The insert and update rule: Data manipulation commands like insert, update, delete must be operational on multiple rows rather than on single row.
8.) The physical independence rule: The database access by users must be independent of changes in storage representation or access methods to data.
9.)The logical data independence rule : The end user application programs or other activities must be independent or must be unaffected when there is a change to the design of the database.
10.) Integrity independence rule : The constraints namely the integrity constraints defined should also be stored in database as data in tables.
11.)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 :
when a distributed version of the DBMS is first introduced; and
when existing distributed data are redistributed around the system.
12.)No subversion rule : If an RDBMS supports a lower level language then it should not bypass any integrity constraints defined in the higher level.