Re: a union is always a join!

>[...] I'm only being precise and basic to justify my
>points clearly. I guess you think I'm too basic. But I think
>that many things you write contradict basics, and that thus
>basics are relevant to my reply.

Absolutely, and it is appreciated, but sometimes you stop there and do not address the actual issue.

[...]

>Along the way you have said (along with a lot of other stuff
>I contradict) that there is a distinction relevant to the user
>between relations that observe changing things, those that
>observe unchanging things and those that derive from
>these;

Not quite. The distinction is between a priori knowledge and a posteriori knowledge. Usually, relations that represent the former are immutable, relations that represent the latter aren't. One working with a database must understand how its relations are to be interpreted, and this distinction is important in that. It is good database design practice to try and keep them apart, by keeping derived information out of the base relations, putting it into queries and views. Of course this distinction can also be made even when D&D's algebra is used. You deny that the user needs to be aware of it.

>and that it is relevant to the user how any of these
>are implemented.

That, too, e.g. performance characteristics of operations may be important.

>And I have said that there isn't. Other
>than what the observing (changing and constant) relations
>represent, to the *user* it's all the same. And treating them
>them the same eases programming.

It eases programming until you wonder
why the program doesn't work.

>philip

-- 
Reinier