Re: One to One Relationship

Frank <fvanbortel_at_netscape.net> wrote in message news:<3E7A22CC.90303_at_netscape.net>...

>
> In that case, you would not model engine as an attribute,
> but as an entity.

Why? I don't need it at all as an entity. I'm not keeping engines as detailed entities themselves. That would be the case if this was an aircraft maintenance system and I was interested in keeping that level of detail. That is not the case.

> Your original (now snipped) post mentioned attributes (plural) engines.

Yes and attributes are not, by definition, instantiated. Entities are.

> 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.

Yes, it is. For their maintenance systems. Not for their flight control and management systems, the detail is not needed. All the flight control people need to know is basic info on status of major ordinance in an aircraft, including engines. That's all.

There is some interesting stuff available on this on the Net. Do a search on ATCCIS and LC2IEDM, that's public info and there is some VERY interesting sub-typing going on in that area!

>
> 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...

Yup, that's exactly what I've done. Used both options, matter of fact. In different places. Just where they were appropriate for the purpose. Which is the bottom line for all this, really: 1:1 is just a special case of sub-typing, nothing wrong with using it where appropriate. Not really restrictd to physical design, it has a place in logical as well.

Cheers
Nuno Souto
wizofoz2k_at_yahoo.com.au.nospam Received on Thu Mar 20 2003 - 19:27:42 CST