Re: Missing entity types

gkiourtas michail wrote:
> -=-=-=-=-=-
>
> Could you please elucidate the following subjects?
> A. One simple way to check for missing entity types involves examining
> each existing entity type to determine if any
> of its attributes has a 'generic key attribute'in its suffix that
> could imply a missing entity type.That is,if an attribute,
> Ai,j of entity type, Ei ,is of the form Att_Generic-Key-Attribute,
> it might be th case that Att should be a new entity type,
> Ek. If so, then a relationship should exist between Ei and Ek.

I have to guess here because it is not clear to me what a 'generic key attribute' or something of the form Att_Generic-Key-Attribute is, but I think they are trying to tell you that if the value of a certain attribute is an identifier then this attribute should actually be an binary relationship between the entity and some other entity type. So if you have the attribute Animal.owner that is a generic key attribute then you should probably introduce an entity type Owner and replace the attribute with the relationship Animal.owned-by.Owner.

I don't know if this does make any sense to you but if it doesn't then I would like to ask you to give me an example of an attribute of the form Att_Generic-Key-Attribute.

> B. A situation where it becomes important to distinguish between
> entities and attributes is when an attrribute Ai,j, of
> entity type, Ei, is the name of another entity type, Ek.
> When this occurs, two possibilities exist:
> 1. The entity type, Ek, is needed for the unique identification of
> Ei. Then Ai,j , should be replaced by the primary
> key attribute(s) of Ek.
> 2. The entity type, Ek, is not needed as part of the key of Ei.
> Then, the attribute is really suggesting a relationship
> between the two entity types.

If you have the attribute Order.client (the client who made the order) and you also have the entity type Client then there is probably something wrong. You should then replace this attribute with the relationship Order.by.Client, except when the attribute is a part of the key. In the latter case you replace it it with the key of Client. So, of the entity type Client has the key Client.client-number then you replace Order.client with Order.client-number.

By the way, I don't agree with the exception that your book makes here. You should always replace the attribute with a relationship. But that is just my opinion. I am getting curioser and curioser about what book you are using. Would you like to tell me?

Kind regards,

    Jan Hidders Received on Sat Nov 18 2000 - 00:45:15 CET