Re: Please help me to design supertype-subtype ..

krislioe_at_gmail.com wrote:
> Hi all,
>
> We are designing our entity, I am confused about the criteria to
> determine whether my two entity should be put into Supertype/Subtype
> relationship.
>
> 1)Just because two entities have common attributes, is it enough reason
> to create supertype-subtype on them ? What are other reason ?

First, many people are unaware the entity relationship modeling can include sub-types and super-types. But it is a very valid and often not used enough categorization.

Generally supertypes and subtypes are things that belong together in the sense of being logically of the same type in some manner. For example if the concept is people then potentially employees and customers could be sub types of people.

When designing for relational databases though many of us want to/tend to separate things that the business operates very differently on. The processes and types of information that you might want to track about customers versus employees can vary widely.

>
> 2)For example, I have two entities : Sales & SalesReturn. They have
> attributes in common. Should I make them supertype-subtype ?

To me at least, selling things is very different than having a customer or distributor return things. You obviously want to minimize the amount of returns.

>
> 3) Does it make any sense to store Sales & SalesReturn in one table ?

I don't think so but it doesn't mean that someone else would not do that.

What does your concept/entity sales represent? Usually you have to sell specific items ( line items ) that are rolled up into a purchase order.

If each item creates it's own purchase that doesn't sound right to me.

>
> Thank you very much,
> xtanto

Have fun. Received on Tue Oct 03 2006 - 10:54:40 CDT