- 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.
- 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.
- 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.
- Active On-line catalog based on the relational model:
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.
- 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.
- 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.
- 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
- 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.
- 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.
- 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.
- 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.
- 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.