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

On Tue, 03 Oct 2006 08:33:37 -0700, krislioe 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 ?

No, it is not enough.

The only real reason I can identify is that the subtype is actually a special type of the supertype.

Cars and trucks share the same attributes. But cars are not 'a type of truck' and trucks are not a 'type of car'. Instead, both are subtypes of vehicle. So the supertype would be vehicle and the subtypes would be car and truck.

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

Probably not.

I would suggest that both are types of transactions. The supertype could be .transaction'.

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

It depends on a lot more factors. For a more complete discussion, I'd suggest reading 'The Art of SQL' (http://www.oreilly.com/catalog/artofsql/)

-- 
Hans Forbrich   (mailto: Fuzzy.GreyBeard_at_gmail.com)   
*** Feel free to correct me when I'm wrong!
*** Top posting [replies] guarantees I won't respond.