Re: Character string relation and functional dependencies

From: Tegiri Nenashi <TegiriNenashi_at_gmail.com>
Date: Thu, 6 Dec 2007 10:15:03 -0800 (PST)
Message-ID: <a96c04d4-870b-4c82-9ff3-f496ccf28f4a@i12g2000prf.googlegroups.com>

On Dec 6, 9:40 am, rp..._at_pcwin518.campus.tue.nl (rpost) wrote:
> Tegiri Nenashi wrote:
>
> [...]
>
> >Informally these FDs correspond to the length(), instr(), and substr()
> >functions. So instead of talking about the class String with the
> >length(), instr(), and substr() member functions, we can focus on a
> >relation and functional dependencies....
>
> Certainly. But we can't describe the full semantics of strings
> in that way. How do you represent concatenation?

That was not the right representation, consider the relation:

str | prefix | suffix | pos

---

abcd | ab | cd | 2
abac | ab | ac | 2
....

Functional dependencies:

prefix & suffix -> str       (corresponds to concatenation)
prefix -> pos                 (length)
str, pos -> prefix           (prefix substring operation)
str, pos -> suffix           (suffix substring operation)

The combination of the latter two (first extract suffix, and then extract prefix) is the usual substring operation: substring(str, pos, length).

Question: how do we represent the "instr" operator?

> Another difference is that database tables are finite and variable,

Oh, relations in database world are certainly not restricted by finite cardinality. Received on Thu Dec 06 2007 - 12:15:03 CST