Re: Is this bad design ?

"ben brugman" <ben_at_niethier.nl> wrote in message news:404c9b34$0$281$4d4ebb8e_at_read.news.nl.uu.net...
> Mother table.
> fields:
> mother_id
> still_valid boolean
> etc_fields
>
> Child table
> fields:
> mother_id
> still_valid boolean
> more_fields
>
>
> The child.still_valid field always contains a true, because only mothers
> withs are still_valid can have children, to enforce this; a relation is
> defined on the fields mother_id and still_valid.
> (A child can not exist without its mother).
>
> Because a field in the child table always has the same value, I think
> there is something wrong. But if this field is taken away a mother might
> become un_valid by another action.
>
> Is this bad design ?
> what is a alternative ?
>
> (As I understand constraints should be implemented in the RDBMS and
> not in the code).
>
> ben brugman

This data could be modeled as:
Mother Table
mother_id
still_valid boolean
etc_fields
child* (* meaning can be multivalued)
  more_fields

If your database permits such modeling (no matter how it actually implements this), then the desired constraint is built in, including knowing whether the mother is still_valid and also getting deleted if the mother is deleted. Hand data with such relationships to any secretary in the 1950's and they will know how to "model" it and store the data for easy retrieval in their paper system. Thank goodness they were not constrained by some desire to "normalize" the data, eh?

smiles. --dawn Received on Wed Mar 10 2004 - 04:21:28 CET