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codds_rules.htm
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<HTML><HEAD><TITLE>Codd's 12 Rules</TITLE></HEAD>
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<B>Ted Codd and his Rules for Relational Databases</FONT>
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<FONT COLOR=blue SIZE=4><B>THE RULES :</B></FONT>
<PRE><FONT COLOR=blue FACE=Verdana SIZE=2>This is C.J Date's version of the 12 Rules from AN INTRODUCTION TO DATABASE SYSTEMS
(5th edition) as set forth in pages 389 - 393
0. (Yes, there is a Rule 0!)
For a system to qualify as a RELATIONAL, DATABASE, MANAGEMENT system, that system must
use its RELATIONAL facilities (exclusively) to MANAGE the DATABASE.
1. The information rule
The information rule simply requires all information in the database to be represented
in one and only one way, Namely by values in column positions within rows of tables.
2. The guaranteed access rule
This rule is essentially a restatement of the fundamental requirement for primary keys.
It says that every individual scalar value in the database must be logically addressable
by specifying the mane of the containing table, the name of the containing column and the
primary key value of the containing row.
3. Systematic treatment of null values
The DBMS is required to support a representation of "missing information and inapplicable
information" that is systematic, distinct from all regular values
(for example, "distinct from zero or any other number," in the case of numeric values),
and independent of data type.
It is also implied that such representations must be manipulated by the DBMS in a
systematic way.
4. Active online catalog based on the relational model
The system is required to support an online, inline, relational catalog that is
accessible to authorized users by means of their regular query language.
5. The comprehensive data sublanguage rule
The system must support a least one relational language that
(a) has a linear syntax,
(b) can be used both interactively and within application programs,
and (c) supports data definition operations (including view definitions),
data manipulation operations (update as well as retrieval),
security and integrity constraints, and transaction management
operations (begin, commit, and rollback).
<IMG ALIGN=left ALT="Grand Canyon" SRC="images/philg/grand-canyon-9.2.jpg">
6. The view updating rule
All views that are theoretically updatable must be updatable by the system.
7. High-level insert, update, and delete
The system must support set-at-a-time INSERT, UPDATE, and DELETE operators.
8. Physical data independence
Self-explanatory.
9. Logical data independence
Self-explanatory
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.
11. Distribution independence
Existing applications should continue to operate successfully
(a) when a distributed version of the DBMS is first introduced;
(b) when existing distributed data is redistributed around the system.
12. The nonsubversion rule
If the system provides a low-level (record-at-a-time) interface, then that
interface cannot be used to subvert the system (e.g.) bypassing a relational
security or integrity constraint.
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