Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]

From: Jan Hidders <>
Date: Wed, 15 Jun 2005 22:32:41 GMT
Message-ID: <d02se.121786$>

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.

Are you sure you are not talking about the (rather old) idea that is commonly known as "the universal relation assumption" or "the universal-relation data model"? For a few references see:

For a very short explanation and a link to Jeffrey Ullman's sheets:

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

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)

Kent, 1981: W. Kent, "Consequences of assuming a universal relation", in ACM Trans. Database Syst., vol. 6, no. 4, pp. 539-556, 1981

Ullman, by the way, didn't agree with Kent and an interesting exhange followed:

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.

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
Received on Thu Jun 16 2005 - 00:32:41 CEST

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