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Re: Is this bad design ?

From: Tony <andrewst_at_onetel.net.uk>
Date: 10 Mar 2004 02:55:07 -0800
Message-ID: <c0e3f26e.0403100255.7a3f6f1d@posting.google.com>


"Dawn M. Wolthuis" <dwolt_at_tincat-group.com> wrote in message news:<c2m1ju$jrb$1_at_news.netins.net>...
> "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

Dawn, what would you do if we extended the example and said that there is also another table called Father, and that every Child must also have a Father? The Child can't be modelled as an attribute of both tables, can it? And then another table called School, and another called Gang, ..., all with similar 1:M relationships. What then? Received on Wed Mar 10 2004 - 04:55:07 CST

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