Dr. Codd is the founder of relational database model. He published a two-part article that lists 12 rules (or principles). These rules are known as Codd's 12 rules. Dr. Codd's rules provide very useful measure (yardstick) for evaluating a relational database system. These rules are used to determine whether a DBMS is relational and to what extent it is relational. Dr. Codd also mentioned that according to these rules, no fully relational system is available yet. The Codd's rules are described below:
0, Rule Zero: According to this rule, any system that is claimed to be Relational Database Management System (RDBMS), must be able to manage the data of database through its relational capabilities.
1. Information Representation: All information in a relational database is represented clearly in the form of rows and columns. It means information is stored in tables.
2. Guaranteed Access: Each and every atomic value in a relational database is guaranteed to be logically accessible by specifying the table name, primary key value and column name.
3. Representation of Null Values: The system must be able to represent null values in a systematic way.
4. Relational Catalog: The system catalog that contains the logical description
of the database must be represented in the same way as ordinary data.
5. Comprehensive Data Sub-language: A relational system may support several languages and various modes of terminal use. However, there must be at least one language whose statements are expressible in some well-defined syntax as character strings and which has ability to support all of the following:
* data definition
* view definition
* data manipulation
* integrity constraints
* transaction boundaries
6. Updating Views: All views that are theoretically updateable are also updateable by the system.
7. Insert, Update and Delete Operations: The system must have capability to insert, update and delete data of tables.
8. Physical Data Independence: Application programs and terminal activities remain logically uniform whenever any changes are made in either storage representation or access methods.
9. Logical Data Independence: Application programs and terminal activities remain logically uniform when structure of database tables changes.
10. Integrity Independence: Integrity constraints specific to a particular relational database must be definable in the relational data sub-language and storable in the catalog, not in the application programs. A minimum of the following two integrity constraints must be supported.
Entity integrity: No component of a primary key is allowed to have a null value.
Referential Integrity: If foreign key is defined in a relational database, then each value of foreign key must have a matching primary key value in another table of database.
11. Distribution Independence: A RDBMS has distribution independence. It means that if the database is distributed, the application programs and user's commands need not be changed.
12. Non-subversion: If a relational system has a low-level (single record at time) language, that low-level language cannot be used to bypass the integrity rules or constraints expressed in the higher-level (multiple-records at a time) relational language.
0, Rule Zero: According to this rule, any system that is claimed to be Relational Database Management System (RDBMS), must be able to manage the data of database through its relational capabilities.
1. Information Representation: All information in a relational database is represented clearly in the form of rows and columns. It means information is stored in tables.
2. Guaranteed Access: Each and every atomic value in a relational database is guaranteed to be logically accessible by specifying the table name, primary key value and column name.
3. Representation of Null Values: The system must be able to represent null values in a systematic way.
4. Relational Catalog: The system catalog that contains the logical description
of the database must be represented in the same way as ordinary data.
5. Comprehensive Data Sub-language: A relational system may support several languages and various modes of terminal use. However, there must be at least one language whose statements are expressible in some well-defined syntax as character strings and which has ability to support all of the following:
* data definition
* view definition
* data manipulation
* integrity constraints
* transaction boundaries
6. Updating Views: All views that are theoretically updateable are also updateable by the system.
7. Insert, Update and Delete Operations: The system must have capability to insert, update and delete data of tables.
8. Physical Data Independence: Application programs and terminal activities remain logically uniform whenever any changes are made in either storage representation or access methods.
9. Logical Data Independence: Application programs and terminal activities remain logically uniform when structure of database tables changes.
10. Integrity Independence: Integrity constraints specific to a particular relational database must be definable in the relational data sub-language and storable in the catalog, not in the application programs. A minimum of the following two integrity constraints must be supported.
Entity integrity: No component of a primary key is allowed to have a null value.
Referential Integrity: If foreign key is defined in a relational database, then each value of foreign key must have a matching primary key value in another table of database.
11. Distribution Independence: A RDBMS has distribution independence. It means that if the database is distributed, the application programs and user's commands need not be changed.
12. Non-subversion: If a relational system has a low-level (single record at time) language, that low-level language cannot be used to bypass the integrity rules or constraints expressed in the higher-level (multiple-records at a time) relational language.