Codd's 12 Rules

Codd's twelve rules are a set of twelve rules proposed by Edgar F. Codd, a pioneer of the relational model for databases, designed to define what is required from a database management system in order for it to be considered relational, i.e., a relational database management system (RDBMS). These rules can be applied on any database system that manages stored data using only its relational capabilities. The Rules are :

1. Information Rule:

All information in a relational database including table names, column names are represented by values in tables. This simple view of data speeds design and learning. User productivity is improved since knowledge of only one language is necessary to access all data such as description of the table and attribute definitions, integrity constraints. Action can be taken when the constraints are violated. Access to data can be restricted. All these information are also stored 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. User productivity is improved since there is no need to resort to using physical pointers addresses. Provides data independence. Possible to retrieve each individual piece of data stored in a relational database by specifying the name of the table in which it is stored, the column and primary key which identified the cell in which it is stored.

3. Systematic Treatment of Nulls Rule:

The RDBMS handles records that have unknown or inapplicable values in a pre-defined fashion. Also, the RDBMS distinguishes between zeros, blanks and nulls in the records hand handles such values in a consistent manner that produces correct answers, comparisons and calculations. Through the set of rules for handling nulls, users can distinguish results of the queries that involve nulls, zeros and blanks. Even though the rule doesn't specify what should be done in the case of nulls it specifies that there should be a consistent policy in the treatment of nulls.

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. The database administrator's productivity is improved since the changes and additions to the catalog can be done with the same commands that are used to access any other table. All queries and reports can also be done as any other table.

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. User productivity is improved since there is just one approach that can be used for all database operations. In a multi-user environment the user does not have to worry about the data integrity an such things, which will be taken care by the system. Also, only users with proper authorization will be able to access data.

6. View Updating Rule: 

Any view that is theoretically updateable can be updated using the 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. The database can change and grow to reflect changes in reality without requiring the user intervention or changes in the applications. 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. Integrity constraints specific to a particular RDB must be definable in the relational data sub-language and storable in the catalog. At least the Entity integrity and referential integrity must be supported.

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.

According to Dr. Edgar. F. Codd, a relational database management system must be able to manage the database entirely through its relational capabilities.

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