E.codd designed 12 rules that stated rules for database to be relational.
1. The Information Rule:For a database to be relational all information is represented as data values in tables.
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 namely
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. Distribution Rule:Irrespective of data being distributed applications must be able to run. That is on other words applications running on the database must be unaffected when the data in database becomes distributed.
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.