Dr. E. F. Codd's 12 rules
for defining a fully relational database
Note that based on these rules there is no fully relational database
management system available today. In particular, rules 6, 9, 10, 11 and 12 are
difficult to satisfy.
- Foundation Rule
A relational database management system must
manage its stored data using only its relational capabilities.
- Information Rule
All information in the database should be
represented in one and only one way - as values in a table.
- Guaranteed Access Rule
Each and every datum (atomic value) is
guaranteed to be logically accessible by resorting to a combination of table
name, primary key value and column name.
- Systematic Treatment of Null Values
Null values (distinct
from empty character string or a string of blank characters and distinct from
zero or any other number) are supported in the fully relational DBMS for
representing missing information in a systematic way, independent of data
type.
- Dynamic On-line Catalog Based on the Relational Model
The
database description is represented at the logical level in the same way as
ordinary data, so authorized users can apply the same relational language to
its interrogation as they apply to regular data.
- Comprehensive Data Sublanguage Rule
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, per some
well-defined syntax, as character strings and whose ability to support all of
the following is comprehensible:
- data definition
- view definition
- data manipulation (interactive and by program)
- integrity constraints
- authorization
- transaction boundaries (begin, commit, and rollback).
- View Updating Rule
All views that are theoretically
updateable are also updateable by the system.
- High-level Insert, Update, and Delete
The capability of
handling a base relation or a derived relation as a single operand applies nor
only to the retrieval of data but also to the insertion, update, and deletion
of data.
- Physical Data Independence
Application programs and terminal
activities remain logically unimpaired whenever any changes are made in either
storage representation or access methods.
- Logical Data Independence
Application programs and terminal
activities remain logically unimpaired when information preserving changes of
any kind that theoretically permit unimpairment are made to the base
tables.
- Integrity Independence
Integrity constraints specific to a
particular relational database must be definable in the relational data
sublanguage and storable in the catalog, not in the application
programs.
- Distribution Independence
The data manipulation sublanguage
of a relational DBMS must enable application programs and terminal activities
to remain logically unimpaired whether and whenever data are physically
centralized or distributed.
- Nonsubversion Rule
If a relational system has or supports a
low-level (single-record-at-a-time) language, that low-level language cannot
be used to subvert or bypass the integrity rules or constraints expressed in
the higher-level (multiple-records-at-a-time) relational
language.
REFERENCES
Codd, E. (1985). "Is Your DBMS Really Relational?" and "Does Your DBMS Run By
the Rules?" ComputerWorld, October 14 and October 21.
Elmasri, R., & Navathe, S. (1994). Fundamentals of Database
Systems. 2nd ed. Redwood City, CA: The
Benjamin/Cummings Publishing Co. pp. 283 – 285.
Source: http://www.cis.ohio-state.edu/~sgomori/570/coddsrules.html