Re: Unique Index

Tino Wildenhain wrote:
> Hi,
>
> Am Mittwoch, den 19.01.2005, 15:02 -0800 schrieb J. Greenlees:
>
>>Roman Neuhauser wrote:
>>
>>># alex(at)meerkatsoft(dot)com / 2005-01-20 01:35:32 +1100:
>>>
>>>
>>>>i have a unique index on a table over multiple columns. If now one of
>>>>the records has a null value in one of the indexed columns i can insert
>>>>the same record multiple times.
>>>>
>>>>Is this a problem within postgres or expected?
>>>
>>>
>>> In SQL, NULL means "unknown value". How could you assert that two
>>> NULLs are equal?
>>>
>>
>>which doesn't make mathematical sense.
>>mathwise null is an empty result.
>>so setting the logic up using the math logic, null values are always equal.
>
>
> What kind of mathematics you are speaking?
> For example you have "infinity" where infinity is never
> equal to infinity.
> Same with null. Which is "unknown" or "undefined"
> So if x is undefined and y is undefined you cannot
> assume x=y - because if you assume this, then
> they would not be undefined anymore.
>
> q.e.d.
>
> Regards
> Tino
>
>
or null as in empty.
an empty result set is a null set, zero results.
declare a variable, but never assign a value, it has a default value of
null from the declaration.
( basically any content of memory space allocated that was not actually
empty is the content, but it's a null value to the app. )

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