>>>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

Not always, remember the mantra? "null is a state, not a value".

state <> unknown.

We can know the state: it's either NULL or NOT NULL.

>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.

:-)


Oh, thy lovely NULL.

Martijn Tonies
Database Workbench - development tool for Firebird and more!
Upscene Productions
http://www.upscene.com
My thoughts:
http://blog.upscene.com/martijn/
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