Alan McDonald wrote:

>>
>>Often used example:
>>I don't know how much money you've got in your wallet, so the value is
>>NULL (unknown).
>>
>>You don't know how much money I've got in my wallet, so it's NULL, too.
>
> I've never thought of this as a good example.
> I have money in my wallet - it could be nothing or it could be $5 or $10 etc
> In the DB, if the field state is NULL - it NEVER has an unknown value in it
> of 5 or 10 - it has no value it's state is NULL. if it has 5 or 10 in it
> then it is NOT NULL.

But the key here is that NULL means unknown (to others than the wallet
owner in the above example). So, to the wallet owner, the amount of
money in his/her wallet is NOT NULL (unless he/she hasn't counted it for
a while...), but to the rest of the world, it's NULL.

Kjell
--
--------------------------------------
Kjell Rilbe
Adressmarknaden AM AB
E-post: kjell.rilbe@...
Telefon: 08-761 06 55
Mobil: 0733-44 24 64