Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
Date: 28 Jun 2005 07:56:05 -0700
[coalescing two messages.]
Jan Hidders wrote:
> Marshall Spight wrote:
> Me personally? It raises all kinds of new and intricate research
> questions that seem mathematically challenging. That doesn't mean that I
> don't think XML is practically useful, on the contrary, but this is what
> primarily fascinates me about it.
Hmmm. I detect a hint of "I like it because it's complicated." That concerns me.
> Oh yes. Everything you mention above is in there, including key
> constraints. The typing system is also very elaborate, it even allows
> you to define certains sets of strings described by regular expression
> as strings, and is user-extensible.
Does that mean it's not statically typed? I don't see how you could statically type whether a string belonged to a regular expression or not, at least not if you wanted to have regular strings be assignable at runtime.
> Let me mention two small things. In the RM if you want to add a
> one-to-many relationship between two entities you have to extend one of
> the relations with a foreign key. If there are more than one candidate
> key you have to choose one of them. In an ER model you don't have to
> make such a choice, you simply indicate that there is a relationship.
Uh, how is that going to work? If I have a user id and an email address as keys, and some user-level detail data (specified as many-to-one with users) and I don't specify a key, what happens when I change the email address key? I don't see how you can get away with not picking one and still having the generality you get if you do pick one.
> Another small thing is updating primary keys. If a primary key has
> accidentally been entered wrong and you want to fix that with an update
> then it is usually not possible to simply update it, and the problem
> gets even worse if it is also refered to by foreign keys. In an ER model
> this is a non-problem.
Would these issue be solved with opaque keys? I feel that one problem
with SQL is that the keys are often integers, which they really
be since it doesn't make sense to add and subtract keys.
I'd like to read more about this. It seems like there are some acm papers on this; I can try to get some of them. Any favorites?
Marshall Received on Tue Jun 28 2005 - 16:56:05 CEST