Re: One to One Relationship

From: Frank <fvanbortel_at_netscape.net>
Date: Thu, 20 Mar 2003 21:21:32 +0100
Message-ID: <3E7A22CC.90303@netscape.net>

Nuno Souto wrote:
> Frank <fvanbortel_at_netscape.net> wrote in message news:<3E78C0BC.4000109_at_netscape.net>...
>

>>
>>That would be 2 instantiations of the attribute engine.

>
>
> Not at all. Attributes are not instantiated. Entities are.
> So at the most, it would be two instantiations of entity engine,
> possibly linked to a particular aircraft. But here is where
> reality hits: each engine in an aircraft is a different part
> of the aircraft, as far as for example maintenance is concerned.
> In the case of an FA18, you got left and right engine. Each
> with different maintenance rates, timings, etc. We can of
> course separate into its own entity, but we'd need two, one
> for each engine "side". Or a sub-type. Which turns it into
> a 1:1. I won't even go into a P3C, where even each engine
> can be of different make... :D
>
> Cheers
> Nuno Souto
> wizofoz2k_at_yahoo.com.au.nospam

In that case, you would not model engine as an attribute, but as an entity.
Your original (now snipped) post mentioned attributes (plural) engines. Engine now being an entity, 'side'(allowable values 'inner left', 'outer left','inner right', 'outer right' or something similar) would be the attribute to distingish between the enigines (instantiations of...). A B52 would fit in that, of course... Engine number might be better, iirc that's what the Airforce folk use themselves.

And yes, you could subtype as well. Using seperate tables (exclusive or triggers needed), or discriminator columns (and views, using the distinct values of the discriminator column), or...

-- 
Regards, Frank van Bortel

Received on Thu Mar 20 2003 - 14:21:32 CST