Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
Date: Fri, 17 Jun 2005 12:53:36 +0200
Jon Heggland schrieb:
> In article <42b28fe7$1_at_news.fhg.de>, email@example.com says...
>>Let's take three tables E, D, and M and two subtables ED and DM. If we >>impose constraints on M then we are not able to infer anything on E. The >>reason is formally clear: ED and DM are independent (orthogonal) because >>they do not have a common subtable (a formal criterion). This means that >>the canonical semantics (or primitive semantics) is represented as a sum >>of two tables ED and DM without intersection like this one: >> >>emp1, dept1, null, null >>emp2, dept2, null, null >>null, null, dept1, mng1 >>null, null, dept2, mng2
> Is this better than the corresponding notion in the RM, namely the join
> (logical and) of ED and DM?
> emp1, dept1, mng1
> emp2, dept2, mng2
I find the question "is this better" somewhat irrelevant. My goal is to remove such ambiguity in data interpretation. When we design our database we want to know clear what is the formal semantics of data. In the above example the final canonical semantics is represented by 4 column table as I wrote it. Your 3 column table is not better or worse - it simply another database.
We assume that our data model has some canonical representation which we call primitive semantics. It is easier formally to perform all manipulations where the semantics is represented in one and the same form. Assume you have constraints in one place of the model (in table 1) and aslo in another place (table 2). How can you find their influence in the whole data semantics in your model? One way is to represent all the constraints as primitive semantics and then impose them using simple rules.
>>It consists of two isolated sections so it is not a fault of the >>database that it cannot infer anything about employees given information >>about managers. We need somehow connect two department columns in the >>middle. In the RM it is not a problem because everything is done >>manually without any rules,
> No, no, no. In the RM, this is done formally, with very definite rules
> indeed. ED and DM are not independent---at least not in the "unrelated"
> sense---they both involve departments. The database knows this (if it is
> properly designed), and it can suggest it to the user, if necessary.
Yes, it can suggest (and it is very useful) but it cannot resolve this problem because there is no formalization for this case.
What I meant when I wrote that in RM we can solve this problem is that we have a complete freedom in manipulating data. Do you agree that in the RM we can do whatever we want with our data by producing result sets which do not make sense? Note I do not say that it is bad or good, I just want to say that this power and freedom to use it compensates our lack of knowledge about data and its meaning.Received on Fri Jun 17 2005 - 12:53:36 CEST