Re: Demo: Modelling Cost of Travel Paths Between Towns

"Alan" <alan_at_erols.com> wrote in message news:2vuglvF2p4mk3U1_at_uni-berlin.de...
>
>
> "Neo" <neo55592_at_hotmail.com> wrote in message
> news:4b45d3ad.0411151728.739fd3ea_at_posting.google.com...
> > > UPDATE your_table
> > > SET age = age + 1
> > > WHERE name = 'john'
> > > AND age IS NOT NULL
> >
> > You are not actually suggesting a solution with NULLs, are you?
> >
> > On page 571, 6th Ed of "An Intro to Db Systems", CJ Date states "NULLs
> > and 3VL are a mistake and have no place in a clean formal system like
> > the relational model".
>
> You're the one who brought it up:
>
> While TM/XDb2 does not have built-in operations such as the above,
> > user can easily create code that will update the age of any thing
> > whose name is john and has an age, including that of things whose
> > class is unknow at design-time (ie a person)
>
> Okay, I won't use a null, I'll use the idea of "unknown" instead. No big
> deal:
> UPDATE your_table
> SET age = age + 1
> WHERE name = 'john'
> AND age > 0
>
> or, depending on how you want to represent the idea of "unknown age" in
the
> database, perhaps
>
> UPDATE your_table
> SET age = age + 1
> WHERE name = 'john'
> AND age = -1
>

The above should be "> -1", not "= -1" (which I don't like as much as > 0, but it depends on what your needs.) Received on Tue Nov 16 2004 - 15:29:31 CET