On Sep 29, 2008, at 4:59 AM, Chris wrote:
Matthew Wilson wrote:
I'm trying to comprehend how NULL values interact with unique
indexes.
It seems like I can insert two rows with NULL values in a column
with a
unique constraint just fine.
Is there something special about NULL? Can anyone post some links to
explain what is going on?
When you think of null as "unknown", it makes sense.
Does an unknown value equal another unknown value?
Also, you wouldn't be able to put a UNIQUE constraint on foreign keys
with a 0..1 to 1 relation if two NULL values would be considered not
unique. That UNIQUE constraint is what makes it a 0..1 to 1 relation
(as would a PRIMARY KEY constraint). Without it it would be a * to 1
relation.
If two NULLs would be considered not unique, only one NULL key
reference would be allowed and all following ones would result in a
unique constraint violation!
Alban Hertroys
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If you can't see the forest for the trees,
cut the trees and you'll see there is no forest.
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