Dr. Edgar Frank Codd was a computer scientist while working for IBM he invented the relational model for database management. Codd.proposed thirteen rules (numbered zero to twelve) and said that if a Database Management System meets these rules, it can be called as a Relational Database Management System. These rules are called as Codd’s12 rules.
Rule Zero:
- The system must qualify as relational, as a database, and as a management system. For a system to qualify as a relational database management system (RDBMS), that system must use its relational facilities (exclusively) to manage the database.
- The other 12 rules derive from this rule. The rules are as follows :
Rule 1:
The information rule:
All information in the database is to be represented in one and only one way, namely by values in column positions within rows of tables.
Rule 2:
The guaranteed access rule:
All data must be accessible. This rule is essentially a restatement of the fundamental requirement for primary keys. It says that every individual value in the database must be logically addressable by specifying table name, column name and the primary key value of the row.
Rule 3:
Systematic treatment of null values:
The DBMS must allow each field to remain null (or empty). The NULL can be represented as “missing information and inapplicable information” that is systematic, distinct from all regular values, and independent of data type. Such values must be manipulated by the DBMS in a systematic way.
Rule 4:
Active online catalog based on the relational model:
The system must support an online, inline, relational catalog that is accessible to authorized users by means of their regular query language
Rule 5:
The comprehensive data sublanguage rule:
The system must support at least one relational language that
- Has a linear syntax
- Can be used both interactively and within application programs,
- Supports data definition operations, data manipulation operations, security and integrity constraints, and transaction management operations.
Rule 6:
The view updating rule:
All views that can be updated theoretically must be updated by the system.
Rule 7:
High-level insert, update, and delete:
This rule states that insert, update, and delete operations should be supported for any retrievable set rather than just for a single row in a single table.
Rule 8:
Physical data independence:
Changes to the physical level (how the data is stored, whether in arrays or linked lists etc.) must not require a change to an application based on the structure.
Rule 9:
Logical data independence:
Changes to the logical level (tables, columns, rows, and so on) must not require a change to an application based on the structure.
Rule 10:
Integrity independence:
Integrity constraints must be specified separately from application programs and stored in the catalog. It must be possible to change such constraints as and when appropriate without unnecessarily affecting existing applications.
Rule 11:
Distribution independence:
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.
Rule 12:
The non-subversion rule:
If the system provides a low-level (record-at-a-time) interface, then that interface cannot be used to subvert the system, for example, bypassing a relational security or integrity constraint.
