Re: database design method

From: Lauri Pietarinen <>
Date: Mon, 18 Nov 2002 13:20:52 +0200
Message-ID: <>

Jan Hidders wrote:

> >
> >> > - I would suppose that the id's could be generated
> >> >each time the query was executed - what's the problem?
> >>
> >> If it is executed a lot you may run out of them.
> >
> >Surely if that same view (that generates - perhaps abstract -
> >identifiers) is queried many times, it represents each time
> >the values of the execution at that point of time. So if
> >I query it twice it makes no difference whether the
> >identifiers are the same or not, or what the result
> >of the query is.
> >
> >So if I say
> >
> > select * from cust_class
> > and get the answer {(1,1),(1,2),(2,3)}
> >
> >and re-execute it after 10 minutes
> > and get the answer {(1,1),(2,2),(2,3)}
> >
> >that would be perfectly correct because
> >it would just be the result of the query
> >and that's it.

> Yes, what could also happen is:
>  first answer:  {(1,1),(1,2),(2,3)}
>  second answer:  {(2,1),(2,2),(1,3)}
> since a relation is a set and there is no fixed order in which the
> identifiers are generated. This is theoretically a bit annoying; nothing has
> changed in the database, you ask the same question, and yet you get a
> different answer. Another more practical problem is that there may be
> another view that (on the basis of this view) computes certain properties of
> these groups like Avg_income(class_id, avg_inc) that will have computed
> certain properties of these classes. If the identifiers change every time
> you query the view the connection between this tables is lost.


first you say that "you may run out of them [integers]"

then the problem is that different id's get assigned.


We must be assuming that the database is changing all the time anyway, so if we want the class_id to mean something, we would have to have a separate table to describe the meaning of each class.

Anyway: how do abstract identifiers help? Now we have to assign the same abstract indentifier to each class - same problem I suppose?

Lauri Pietarinen Received on Mon Nov 18 2002 - 12:20:52 CET

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