Re: MV counterexample

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"Anthony W. Youngman" <wol_at_thewolery.demon.co.uk> wrote in message news:W02yWkLIi9mAFwaE_at_thewolery.demon.co.uk...
> In message <409b4ddf$1_at_post.usenet.com>, x <x-false_at_yahoo.com> writes
> >> Bearing in mind that apparently, in order to have real data integrity,
> >> we need user-defined primary data types, how on earth is that going to
> >> be PRACTICAL without pushing at least some integrity checks into
> >> user-space.
> >
> >> A database relies on metadata to do integrity checks. Certainly with
> >> current RDBMSs, a major function of analysis is to convert metadata
into
> >> data. Without some way of converting that data back into metadata (ie
> >> giving the user the ability to modify the operation of the database
> >> engine) there is no way the dbms is going to be able to apply any
> >> integrity check that relies on that sort of (meta)data.
> >
> >I don't understand this.
> >Could you be more specific ?
>
> Okay. Simple. Let's take a data type of "sex".
>
> According to ISO, this has four values, namely "male", "female",
> "irrelevant", "unknown".
>
> We can enforce this (do our integrity check) with a trigger - that's
> user space.
>
> Or we can use a foreign key into a lookup table - at which point it
> ceases to be a "user defined data type", because we may have a situation
> where the data consists of a (variable) range and/or special values -
> for example "age" is normally a number but can have the values "unknown"
> or "dead".
>
> If we want to have user-defined datatypes like "age" or "sex", the user
> is going to have to be able to program (in some sense or other) the
> integrity checks, therefore integrity checking must be a user-space
> phenomenon. I just don't see how you can get round it. By definition,
> the database itself cannot use data to do primary integrity checks - it
> needs metadata. But the user can't provide metadata because the database
> itself has no way of understanding it - it'll think it's data. Catch 22.

Well, in the DKNF there are only two checks : - domain membership
- keys enforcement
One of the problems the relational model is supposed to solve is : "In contrast, the problems treated here are those of data independence--the independence of application programs and terminal activities from: GROWTH in data types"
:-)

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