Re: Demo: Modelling Cost of Travel Paths Between Towns

> > > 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?
> >
> > Per CJ Date, "NULLs and 3VL are a mistake
> > and have no place in a clean formal system like the relational model".
>
> Okay, I won't use a null, I'll use the idea of "unknown" instead.

An thus you have introduced 3VL into your system. If john age is unknown, how should the following code below proceed?

if (john.age == mary.age){
  // True
}
else{
  // False
}

If your system evaluates it to either TRUE or FALSE, you may be wrong in some cases. It actually needs a third case "// Unknown".

Per Date, "3VL suffers from the very serious ('showstopper') problem that it does not match reality - that is, results that are correct according to 3VL are sometimes incorrect in the real world. NULLs and 3VL undermine the entire foundation of the relational model."

Attempts to mask NULLs with unknowns or any other value, don't work (you must not have read my prior posts expounding the above point). Received on Wed Nov 17 2004 - 04:31:41 CET