Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
Date: Mon, 20 Jun 2005 07:31:24 GMT
> "Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
>> >>After looking a bit longer at your model I think FDM might be especially >>relevant,
> Are you talking about the FDM (Shipman) as reperesented by the Daplex
> If so, my recollection is that the FDM is not much different
> from the network/OODB approach in which case the argument can be reduced to
> "the network model/OODB contra the RM".
Yes and no. Although the data models look roughly the same, the term "network model" carries with it a few assumptions about how it should be implemented, and these no longer apply. So it may look similar at the surface, but it is actually very different.
> I am sure you know that Daplex,
> btw, is a navigational language, not a declarative one.
It's no more or less declarative than, say, first order logic is. There is in both cases a naive implementation model one might consider navigational but there is an easy and straighforward translation to an implementation model that works at set-level. Compare this also to the relationship between the nested relational calculus (NRC) and the nested relational algebra (NRA) as defined by Limsoon Wong et al. One could probably argue that NRC is navigational and NRA is declarative but the two are straightforwardly translatable into each other, so such a statement is largely superficial and at a deeper level simply doesn't make much sense.
>>and probably even more so in it's modern incarnation that is the data model >>for Description Logics.
> I am not sure how the old FDM is related to the Description Logic, the
> latter being a function free subset of FOL with at most three variable
> names (that is if our vocabularies coincide).
There are many different types of Description Logics, but I would in general say it is more like the two-variable fragment. For example, the satisfiability of the three-variable fragment is undecidable, and DLs usually are decidable. On the other hand many DLs actually go a bit beyond FO^2.
Anyway, the relationship between them is that their data models are essentially the same: binary relations over possibly abstract domains.
- Jan Hidders