Re: Is it a prorperty or entity

In article <lgajgu4vg6pu0htb6jr471tce43peau96l_at_4ax.com>, GoranG <no_at_spam.net> wrote:
>
>Are rules here simply:
>a) if meaning of other properties depend on a certain property (within
>problem specifications)

If the meaning of a property depends on something then it is not really a single property but should be split into a separate property for every distinct meaning. This may mean that you get optional properties, i.e., properties that are sometimes undefined. If this happens and you get big clusters of properties that are always together defined or undefined then this is a sign that a subtype should be introduced. E.g., if entity E has properties a, b, c, d and e, of which a, b, and c are always defined and d and e are always together defined or undefined then you introduce the subtype E' with properties d and e and let E have only the properties a, b and c.

>or
>b) if meaning (or even type) of a relationship depends on a certain
>property (within ...)
>then divide entity on this property?

Also here: if the meaning is different you have to model it as a different relationship. Don't forget you are doing conceptual modelling here, and not implementation. Mixing things with different meanings into the same column or table is for later. Conceptual modelling is about making things clear, not confusing them.

• Jan Hidders