Model

According to Date: online.
Articles by Christopher J Date about how the relational model can fulfill all OO needs. Web-only articles from DBPD that never made to print.
http://www.dbpd.com/vault/new_online.shtml#ATD

According to Date: recent.
Articles by Christohper J Date with emphasis on foundations. Web published by DBPD after the print magazine folded.
http://www.intelligententerprise.com/ports/search_date.shtml

Against the Grain
Contrarian articles by Fabian Pascal about the necessity of getting the relational model properly implemented.
http://searchoracle.techtarget.com/tipsIndex/0,289482,sid41_tax301453,00.html

Database Debunkings
Analysis of various systems and ideas floating in the database scene, against the standards set by the relational model for database management.
http://dbdebunk.com/

Date’s 12 Rules for DDB's
Presentation slides about C.J. Date's goals for relational databases. Orthogonal to the relational model. [PDF]
http://hamilton.bell.ac.uk/databases/documents/Dates12RulesforDDBs.pdf

David McGoveran
Several short, insightful articles on the relevance of the Relational Model to enterprise application integration.
http://www.bijonline.com/index.cfm?section=author&page=details&auid=14

FirstSQL Issue Papers
Opinions fundamented on the relational model about NULLs, SQL and DBMS
http://www.firstsql.com/issues.shtml#issue

Fondation Matters
Series of relational database model articles by Chris J Date, with emphasis on rules applications. Registration required.
http://www.brcommunity.com/date.php

Introduction to Relational Databases
Concentrating on the 12 Rules from EF Codd and a succint, informal definition of normalisation with examples.
http://www.15seconds.com/issue/020522.htm

Journal of Conceptual Modeling
Bi-monthly journal about conceptual modelling and mainly relational database design.
http://www.inconcept.com/JCM/

A Relational Model of Data for Large Shared Data Banks
The paper that started it all. The first public statement of the Relational Model.
http://portal.acm.org/citation.cfm?id=358007

12 Rules
12 Rules for a relational database by EF Codd.
http://www.cse.ohio-state.edu/~sgomori/570/coddsrules.html

A Simple Guide to Five Normal Forms
A Simple Guide to Five Normal Forms in Relational Database Theory
http://www.bkent.net/Doc/simple5.htm

The Third Manifesto
Additional material related to the book.
http://www.dcs.warwick.ac.uk/~hugh/TTM/

Tuple-Marks
A proposal to avoid default or special values, SQL NULLs and three-valued logic in the representation of missing information.
http://74.125.113.104/search?q=cache:uYQNMQjziPwJ:chimu.com/publications/tupleMarks/+tuple.marks&hl=fr&ct=clnk&cd=1&gl=fr

Twelve Rules for Business Rules
Paper by C.J. Date on business rules as data integrity constraints. [PDF]
http://www.informit.com/content/images/0201708507/samplechapter%5Crules.pdf