Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
Date: Wed, 15 Jun 2005 22:32:41 GMT
Message-ID: <d02se.121786$Ww3.6933636_at_phobos.telenet-ops.be>
Alexandr Savinov wrote:
>
> The first point is that once a column is defined for some table in the
> schema then it *formally* can be considered a completely legal column
> for all other tables.
For a very short explanation and a link to Jeffrey Ullman's sheets:
http://app.deklarit.com/kb/article.aspx?id=10038&cNode=8J8X2Y
For an easy-to-understand exposition and motivation see: (sorry for the bad scan)
Moshe Y. Vardi, The Universal-Relation Data Model for Logical Independence, IEEE Software, v.5 n.2, p.80-85, March 1988 http://citeseer.ist.psu.edu/552794.html
For some explanations why the idea is not as simple as might seem at first sight, and not without problems:
Ronald Fagin, Alberto O. Mendelzon, Jeffrey D. Ullman: A Simplified
Universal Relation Assumption and Its Properties. ACM Trans. Database
Syst. 7(3): 343-360 (1982)
http://www.almaden.ibm.com/cs/people/fagin/tods82.pdf
Kent, 1981: W. Kent, "Consequences of assuming a universal relation", in ACM Trans. Database Syst., vol. 6, no. 4, pp. 539-556, 1981 http://plantijn.cmi.ua.ac.be/~hidders/pubs/UniRel/Kent81.pdf
Ullman, by the way, didn't agree with Kent and an interesting exhange
followed:
http://plantijn.cmi.ua.ac.be/~hidders/pubs/UniRel/Ullman83.pdf
http://plantijn.cmi.ua.ac.be/~hidders/pubs/UniRel/Kent83.pdf
For another thorough exposition:
D. Maier, J. D. Ullman, and M. Y. Vardi. On the foundation of the
universal relation model. ACM Trans. on Database System (TODS),
9(2):283-308, 1984.
http://plantijn.cmi.ua.ac.be/~hidders/pubs/UniRel/MaUlVa84.pdf
Today the general opinion seems to be that it's an interesting idea for a user-interface and has it's merits, for example for data-integration and OLAP, but it makes certain easy queries easier and certain hard queries much harder, and it can make data-modeling simpeler in some cases but also a lot more complicated in others, esp. if your schema tends to be cyclic (see the articles for what that means). So it has its uses, but is certainly no replacement for the relational model.
- Jan Hidders