From: gkiourtas michail Newsgroups: comp.databases.theory Subject: Re: Missing entity types Date: Sat, 18 Nov 2000 14:54:16 +0200 Organization: National Technical University of Athens, Greece Lines: 65 Message-ID: <3A167BF8.7E34DBC6@otenet.gr> References: <3A1456BE.56ADE281@otenet.gr> <8v4fub$f3t$1@news.tue.nl> NNTP-Posting-Host: athe530-i149.otenet.gr Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Trace: ulysses.noc.ntua.gr 974551955 42008 195.167.111.149 (18 Nov 2000 12:52:35 GMT) X-Complaints-To: abuse@ntua.gr NNTP-Posting-Date: 18 Nov 2000 12:52:35 GMT X-Mailer: Mozilla 4.6 [en] (Win95; I) X-Accept-Language: en,el Jan Hidders wrote: > 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 Yes.It is not a textbook.It is the article "Relational database design based on the Entity-Relationship model" by Storey DATA &AND KNOWLEDGE ENGINEERING VOL 7 pages 47-83 Thanks for your help.