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Problem with E-R model

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  Problem with E-R model "Tony Johansson" <johansson.andersson@telia.com> - 2015-02-06 18:40 +0100
    Re: Problem with E-R model Erland Sommarskog <esquel@sommarskog.se> - 2015-02-06 20:44 +0100

#1876 — Problem with E-R model

This is a question about horse jumping.

I have these relations one Rider can have many horses but one horse can only 
be owned by one rider.So here we have one to many counted from the Rider 
side.

One Rideclub can have many member which is Riders but one Rider can belong 
to only one RideClub. So here we have one to many counted from Rideclub

One Rider can participate in many classes and one class can have many Rider 
so here we have many to many.


For example
Nisse Hult owns the two horses Trisdan and Sylfid.

In my current E-R design I can tell which horse the Rider is using in a 
special class I can only know that Nisse Hult is using any of the horses 
that he owns.

If I for example change the conditions so that I have one to one relationhip 
between Rider and Horse that will solve the problem
because if Hisse Hult is starting in a class he owns only one horse.

The other solution is that I can create a new table with is the contents of 
Horse and Rider and then remove the  Horse and Rider.
But if I do so I get a lot of double information as you can see here.
So if I do this I can see directly which horse Nisse Hult is using becuse 
the Id will be put in the relation table in between
Rider and Class

Id   social sec number        Rider          Horse       ImgHorse 
DescHorse               ImgRider              RiderDesc          Phone
 1  123456-0001               NisseHult   Trisdan    ImgTrisdan    Info 
about Trisdan     img Nisse Hult   Info Nisse Hult   123 45
 2  123456-0001               NisseHult   Kitty        ImgKitty        Info 
about Kitty        Img Nisse Hult   Info Nisse Hult    123 45

I hope you can understand what I'm trying to say.

So what is the best solution if I want to know which Horse the Rider is 
starting with in a Class?

//Tony

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#1877

Tony Johansson (johansson.andersson@telia.com) writes:
> I have these relations one Rider can have many horses but one horse can
> only be owned by one rider.So here we have one to many counted from the
> Rider side. 

From what I have understood, a horse can be co-owned by several people.
And the rider does not necessarily own the horse at all. But it might
be that a horse typically only has one rider.

> For example
> Nisse Hult owns the two horses Trisdan and Sylfid.
> 
> In my current E-R design I can tell which horse the Rider is using in a 
> special class I can only know that Nisse Hult is using any of the horses 
> that he owns.

Not sure that I follow. If you tie the horse to the class, then you 
also know the rider. As long as we can assume that the horse can 
only have one rider. Since this is not my business domain, I am not
sure how safe this assumption is. Else you would need both horse, rider and 
class to be the key in that table.
 
> The other solution is that I can create a new table with is the contents
> of Horse and Rider and then remove the  Horse and Rider. 
> But if I do so I get a lot of double information as you can see here.

Right, as you have realised that is not the way should go.


-- 
Erland Sommarskog, Stockholm, esquel@sommarskog.se

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