Re: How will schemas be affected by nested relations?

Marshall wrote:
> How will schema design be affected by having the ability to use
> nested relations? I have an intuition that it might not be that
> much; that nesting is a *little* useful but not all that *much* useful.
> However, I am concerned that I don't have a normal form to
> inform design choices.
>
> When should relation schemas be nested?

Does Darwen's GUNF from the TTM email list cover this marshall?

To quote Jon Heggland in:
http://groups.google.com/group/comp.databases.theory/msg/16c6debecdb29896?dmode=source

"Let rv be a relvar with an attribute rva that is of a relation type with attributes a1, ..., an. Then rv is in GUNF if and only if there exists a possible value r of rv such that there does not exist a relation s such that:
r WHERE COUNT(rva) > 0 = s GROUP ( { a1, .., an } AS rva )"

>
> I have an idea that the answer might relate to when we use
> ON DELETE CASCADE. If a logical entity has no existence
> without the "enclosing" context of another entity, then it might
> be nested. Another consideration is whether it needs to
> be referred to directly.
>
> For example, an invoice line item without an associated
> invoice doesn't make much sense. However, an invoice
> is probably something we want to keep even if for some
> reason we delete the associated customer. So I can see
> invoice line items as being a candidate for nesting, but
> not invoices.
>
> However I would be much happier if I had a more formal
> treatment than this. Anyone have any references?
>
>
> Marshall
Received on Tue Jul 25 2006 - 01:37:57 CEST