Re: Authoritative References

"Marshall Spight" <mspight_at_dnai.com> wrote in message news:wU58d.172724$D%.67806_at_attbi_s51...

> Cool! So suddenly I'm less clear on what the essential
> difference between a relation and a function is. Now
> it seems like it's just two ways of looking at the same
> thing, since both must have a key, and may have more
> than one. Perhaps the differences is simply that of
> saying (n, m) for the relation and n -> m for the
> function. Hmmm.....

You probably already know all this, but it's better to make it explicit.

Going back to the math, a function has only one value, a relation does not necessarily. Thus Cosine is a function, but Arc Cosine is a relation. That is, there is a whole set of angles whose cosine have the same value.

Now let's go back to data. "Find all the employees who live in Nashua, NH."

In the most usual design of personnel data, it's going to look like this:

select employee_id, first_name, last_name from employees
where city = 'NASHUA' and state = 'NH'

Now, the fact that we are working with a relation here, rather than a function, is predicated on the fact that the result is a set, not an individual employee. There is a unique key here, but it didn't determine the answer.

If you restrict your self to a functional view, "Where does employee with id = 12345 live?" is an easy question to phrase. But the previous question is not. Received on Mon Oct 04 2004 - 12:51:50 CEST