or null as in empty.Hi,
Am Mittwoch, den 19.01.2005, 15:02 -0800 schrieb J. Greenlees:
Roman Neuhauser wrote:
# alex@xxxxxxxxxxxxxxx / 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
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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